wrap container run in try/except block

Captures proper stderr/stdout messages so we can better
diagnose why some commands seem to fail for users.

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
Merge pull request #173 from Ubuntu-Solutions-Engineering/patch-capture-ssh-errors

wrap container run in try/except block
Merge branch 'master' into patch-install-new-maas