node-group probe-and-enlist-hardware model=vmware does not return errors

probe-and-enlist-hardware does not surface an error exit code when the enlistment fails due to invalid credentials or invalid host.

# Use invalid vmware workstation host IP 192.168.2.7
$ maas admin node-group probe-and-enlist <clusterID> model=vmware accept_all=true prefix_filter=cloud username=csmith password=<valid_password> host=192.168.2.7
Success.
Machine-readable output follows:

# Use valid vmware workstation host IP 192.168.2.75 and invalid credentials
$ maas admin node-group probe-and-enlist <clusterID> model=vmware accept_all=true prefix_filter=cloud username=junk password=<valid_password> host=192.168.2.5
Success.
Machine-readable output follows:

# Maas logs

# packages
ii maas 1.8.2+bzr4041-0ubunt all MAAS server all-in-one metapackage
ii maas-cli 1.8.2+bzr4041-0ubunt all MAAS command line API tool
ii maas-cluster-controller 1.8.2+bzr4041-0ubunt all MAAS server cluster controller
ii maas-common 1.8.2+bzr4041-0ubunt all MAAS server common files
ii maas-dhcp 1.8.2+bzr4041-0ubunt all MAAS DHCP server
ii maas-dns 1.8.2+bzr4041-0ubunt all MAAS DNS server
ii maas-proxy 1.8.2+bzr4041-0ubunt all MAAS Caching Proxy
ii maas-region-controller 1.8.2+bzr4041-0ubunt all MAAS server complete region controller
ii maas-region-controller-min 1.8.2+bzr4041-0ubunt all MAAS Server minimum region controller
ii python-django-maas 1.8.2+bzr4041-0ubunt all MAAS server Django web framework
ii python-maas-client 1.8.2+bzr4041-0ubunt all MAAS python API client
ii python-maas-provisioningserver 1.8.2+bzr4041-0ubunt all MAAS server provisioning libraries