Code review comment for lp:~mskalka/juju-ci-tools/assess-destroy-model

This looks good, though please update the error message as noted below. Also, Curtis should re-review before you merge to make sure his concerns are dealt with.

A few notes about testing:
assertRaisesRegexp (aka assertRaisesRegex in Python3) is much nicer than assertRaises followed by checking message content.

Also, assertIn should be used in preference to assertTrue(x in y).