Fixed stacky negative smoke tests

Fixed few stacky negative smoke tests which were failing
because the response now returns an object which
doesnt have Error & Message as its attributes.
Asserting on the response.reason for few tests.
Added msg as a class variable to Stacktach fixtures.

Change-Id: Iff86502742907bd5a279d680e54c8e14f4a50757
Merge "Fixed stacky negative smoke tests"