Tests failures on Saucy: some tests in test_commands.py fail with a CommandError.

[Raphaël Badin]

The way call_command() raises error in case the command fails has been changed in Django 1.5.
https://docs.djangoproject.com/en/dev/releases/1.5/#minor-features:
"""Management commands do not raise SystemExit any more when called by code from call_command. Any exception raised by the command (mostly CommandError) is propagated."""

This is all for the best — I think — but the problem in MAAS is that we want to be compatible with both Django 1.4 and Django 1.5.