Handle non-openstack errors gracefully

The http error is not guaranteed to come directly from an OpenStack
service. Even if it does, it may not conform to the usual structure.
That kind of response should be handled properly anyway.

Change-Id: Ibd4c678d068ca10a53a72520f7eeffcc84133f07
Closes-bug: 1277565
Merge "Handle non-openstack errors gracefully"