[2.2.0+bzr6057-snap] Stacktrace appears when pointing to a non-existent region controller

Adequate error handling is missing from the MAAS snap.

When putting the snap in 'region' mode and pointing to the default MAAS URL a stacktrace can be displayed. The user could instead be presented with something like:

"There is no functional region controller available at the specified URL."

$ snap info maas

.
.
.
tracking: stable
installed: 2.2.0+bzr6057-snap (91) 98MB devmode
refreshed: 2017-05-30 16:15:09 +0000 UTC
channels:
  latest/stable: 2.2.0+bzr6057-snap (91) 98MB -
  latest/edge: trunk+bzr6071-snap (101) 98MB devmode

$ sudo maas init

Mode (all/region+rack/region/rack/noneall/region+rack/region/rack/none) [default=all]? region
MAAS URL [default=http://10.55.60.1:5240/MAAS]:
Traceback (most recent call last):
  File "/snap/maas/91/bin/maas", line 24, in <module>
    sys.exit(run())
  File "/snap/maas/91/bin/maas", line 21, in run
    return maascli.main()
  File "/snap/maas/91/lib/python3.5/site-packages/maascli/__init__.py", line 29, in main
    options.execute(options)
  File "/snap/maas/91/lib/python3.5/site-packages/maascli/snappy.py", line 573, in __call__
    raise exc
  File "/snap/maas/91/lib/python3.5/site-packages/maascli/snappy.py", line 570, in __call__
    self.handle(options)
  File "/snap/maas/91/lib/python3.5/site-packages/maascli/snappy.py", line 669, in handle
    help_text=ARGUMENTS['database_host']['help'])
KeyError: 'database_host'

Unsurprisingly, the same thing happens for mode 'region+rack'.

Oddly, no error is generated at all when 'rack' mode is chosen, even though there is no region controller to connect to:

$ sudo maas init

Mode (all/region+rack/region/rack/noneall/region+rack/region/rack/none) [default=all]? rack
MAAS URL [default=http://10.55.60.1:5240/MAAS]:
Secret: kjkj

$ sudo maas status
dhcpd STOPPED Not started
dhcpd6 STOPPED Not started
ntp RUNNING pid 6064, uptime 0:01:41
rackd RUNNING pid 6066, uptime 0:01:41
tgt RUNNING pid 6225, uptime 0:01:22

Does this mean that the rack controller was installed but is not registered (with a region controller)?