Charm install fails with hook failed: "website-relation-joined"

Landscape-server latest/edge Rev. 58 fails to install with:

```
landscape-postgresql/0* active idle 2 10.246.167.128 5432/tcp Live master (12.12)
  canonical-livepatch/1 active idle 10.246.167.128 Running kernel 5.4.0-131.147-generic, patchState: nothing-to-apply (source version/commit f1e83ae)
  filebeat/1 active idle 10.246.167.128 Filebeat ready.
  landscape-client/0* maintenance idle 10.246.167.128 Need computer-title and juju-info to proceed
  logrotated/1 active idle 10.246.167.128 Unit is ready.
  nrpe/1 active idle 10.246.167.128 icmp,5666/tcp Ready
  ntp/1 active idle 10.246.167.128 123/udp chrony: Ready
  telegraf/1 active idle 10.246.167.128 9103/tcp Monitoring landscape-postgresql/0 (source version/commit d208a64)
landscape-rabbitmq-server/0* active idle 7 10.246.165.7 5672/tcp Unit is ready
  canonical-livepatch/3 active idle 10.246.165.7 Running kernel 5.4.0-131.147-generic, patchState: nothing-to-apply (source version/commit f1e83ae)
  filebeat/3 active idle 10.246.165.7 Filebeat ready.
  landscape-client/2 maintenance idle 10.246.165.7 Need computer-title and juju-info to proceed
  logrotated/3 active idle 10.246.165.7 Unit is ready.
  nrpe/3 active idle 10.246.165.7 icmp,5666/tcp Ready
  ntp/3 active idle 10.246.165.7 123/udp chrony: Ready
  telegraf/3 active idle 10.246.165.7 9103/tcp Monitoring landscape-rabbitmq-server/0 (source version/commit d208a64)
landscape-server-haproxy/0* maintenance idle 5 10.246.164.152 Configuring HAProxy
  canonical-livepatch/2 active idle 10.246.164.152 Running kernel 5.4.0-131.147-generic, patchState: nothing-to-apply (source version/commit f1e83ae)
  filebeat/2 active idle 10.246.164.152 Filebeat ready.
  landscape-client/1 maintenance idle 10.246.164.152 Need computer-title and juju-info to proceed
  logrotated/2 active idle 10.246.164.152 Unit is ready.
  nrpe/2 active idle 10.246.164.152 icmp,5666/tcp Ready
  ntp/2 active idle 10.246.164.152 123/udp chrony: Ready
  telegraf/2 active idle 10.246.164.152 9103/tcp Monitoring landscape-server-haproxy/0 (source version/commit d208a64)
landscape-server/0* error idle 6 10.246.167.51 hook failed: "website-relation-joined"
  canonical-livepatch/0* active idle 10.246.167.51 Running kernel 5.4.0-131.147-generic, patchState: nothing-to-apply (source version/commit f1e83ae)
  filebeat/0* active idle 10.246.167.51 Filebeat ready.
  logrotated/0* active idle 10.246.167.51 Unit is ready.
  nrpe/0* active idle 10.246.167.51 icmp,5666/tcp Ready
  ntp/0* active idle 10.246.167.51 123/udp chrony: Ready, OK: chronyd has only been running 231 seconds
  telegraf/0* active idle 10.246.167.51 9103/tcp Monitoring landscape-server/0 (source version/commit d208a64)
```
The charm configs are as described in https://docs.google.com/presentation/d/1FNQcxTUxRWLmOiz9R_j80wfKi8Gc6JOzZ7L9AlfT2VA/edit#slide=id.g1516ad5ef40_0_53

In the charmlog we see:

```
unit-landscape-server-0: 20:02:19 ERROR unit.landscape-server/0.juju-log website:79: Uncaught exception while in charm code:
Traceback (most recent call last):
  File "./src/charm.py", line 569, in <module>
    main(LandscapeServerCharm)
  File "/var/lib/juju/agents/unit-landscape-server-0/charm/venv/ops/main.py", line 431, in main
    _emit_charm_event(charm, dispatcher.event_name)
  File "/var/lib/juju/agents/unit-landscape-server-0/charm/venv/ops/main.py", line 142, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-landscape-server-0/charm/venv/ops/framework.py", line 316, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-landscape-server-0/charm/venv/ops/framework.py", line 784, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-landscape-server-0/charm/venv/ops/framework.py", line 857, in _reemit
    custom_handler(event)
  File "./src/charm.py", line 367, in _website_relation_joined
    with open(error_file_path, "rb") as error_file:
FileNotFoundError: [Errno 2] No such file or directory: '/opt/canonical/landscape/canonical/landscape/offline/unauthorized-haproxy.html'
unit-landscape-server-0: 20:02:19 ERROR juju.worker.uniter.operation hook "website-relation-joined" (via hook dispatching script: dispatch) failed: exit status 1
```