Avoid AttributeError: SecurityGroup error handle

On the project/access_and_security/security_groups/<detail> page, some
errors inside the
openstack_dashboard/dashboards/
project/access_and_security/security_groups/views
DetailView._get_data method are still resulting in a None being
returned. This causes an error in the DetailView.get_data method as
NoneTypes clearly don't have a .rules attribute.

I'm not sure why the error is sneaking through the exceptions.handle
above, but they most definitely are. Perhaps Ana Krivokapic's changes
here: https://review.openstack.org/#/c/84109/ may eventually solve this,
but it's unclear.

In the mean time, and potentially correct overall, if a None is
returned, the get_data method should return an empty list instead of
throwing an exception.

Change-Id: Ibc3bbb8c1302fca0c8a134abee8b7c649ad8c315
Closes-Bug: 1312192
Fix unicode error when assertMessageCount() fails

When assertMessageCount() fails, we display the list of messages that
were included in the response. However because the messages are lazily
translated by default, this would fail and show the following error
instead:

 TypeError: sequence item 0: expected string, __proxy__ found

Change-Id: I1d2ed97b66428e0ee1aac783c8370ab3ea665b29
Closes-Bug: #1315031
Merge "Avoid AttributeError: SecurityGroup error handle"
Merge "Fix unicode error when assertMessageCount() fails"