return code should not be always 0

In my opinion juju-lint should return different return code if any warning or error was found,
and returning 0 should be only if no issues was found.

An example of misbehavior where I expected to see 1 or a return code other than 0.
```bash
x1:➜ juju-lint git:(1.0.6) ✗ juju-lint -c /snap/juju-lint/current/contrib/fcb-ussuri-focal.yaml -t openstack tests/resources/fcb-yoga-focal-bundle.yaml
2023-05-31 11:30:49 [INFO] juju-lint version 1.0.6 starting...
        * Config directory: /home/rgildein/.config/juju-lint
        * Cloud type: openstack
        * Manual file: tests/resources/fcb-yoga-focal-bundle.yaml
        * Rules file: /snap/juju-lint/current/contrib/fcb-ussuri-focal.yaml
        * Log level: INFO

2023-05-31 11:30:49 [WARNING] Regex autodetection feature of the eq operator is deprecated. It will only check for equality in the future. Please use the search operator for regex checks.
2023-05-31 11:30:49 [INFO] [tests/resources/fcb-yoga-focal-bundle.yaml] Linting manual file...
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application ceph-mon has no config for 'nagios_additional_checks_critical', cannot determine if == True.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application ceph-mon has no config for 'nagios_additional_checks', cannot determine if == '{"ceph_slow_requests": "slow requests"}'.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application ceph-radosgw has no config for 'ceph-osd-replication-count', cannot determine if == 3.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application cinder has no config for 'worker-multiplier', cannot determine if != ''.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application glance has no config for 'ceph-osd-replication-count', cannot determine if == 3.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application glance has no config for 'worker-multiplier', cannot determine if != ''.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application keystone has no config for 'worker-multiplier', cannot determine if != ''.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application neutron-api has no config for 'worker-multiplier', cannot determine if != ''.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application nova-cloud-controller has no config for 'worker-multiplier', cannot determine if != ''.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application nova-compute has no config for 'ceph-osd-replication-count', cannot determine if == 3.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application nova-compute has no config for 'worker-multiplier', cannot determine if != ''.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application ntp has no config for 'auto_peers', cannot determine if == False.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application octavia has no config for 'worker-multiplier', cannot determine if != ''.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application ovn-central has no config for 'ovsdb-server-election-timer', cannot determine if >= 4.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application placement has no config for 'worker-multiplier', cannot determine if != ''.
2023-05-31 11:30:50 [WARNING] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Application rabbitmq-server has no config for 'queue_thresholds', can't search the regex pattern '\\W\\*, \\W\\*, 25000, 27500'.
2023-05-31 11:30:50 [WARNING] endpoint: elasticsearch not found on filebeat
2023-05-31 11:30:50 [ERROR] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Charm 'ubuntu-advantage' missing on machines: ['0', 'lxd:0', '1', 'lxd:1', '2', 'lxd:2', '3', 'lxd:3', '4', 'lxd:4', '5', 'lxd:5', '6', 'lxd:6', '7', 'lxd:7', '8', 'lxd:8', '9', '10', '11']
2023-05-31 11:30:50 [ERROR] [tests/resources/fcb-yoga-focal-bundle.yaml] [manual/manual] Openstack ops charm 'prometheus-grok-exporter' is missing
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'grafana' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'prometheus' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'prometheus' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'grafana' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'prometheus' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'graylog' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'grafana' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'grafana' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'graylog' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'landscape-server' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'nagios' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'nagios' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'prometheus' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'prometheus' is from another model
2023-05-31 11:30:50 [WARNING] Multi-model is not supported yet. Please check if 'prometheus' is from another model [0.69s]
x1:➜ juju-lint git:(1.0.6) ✗ echo $?
0 [0.01s]
```

Potentially we can use same returns codes as we are using w/ nrpe checks. So 0 - ok, 1 - any warning,
2 - any error and 3 - other?