Add negative tests to api

1. Add couple of negative tests, find the way, more to go.

2. Add assertNotFound to the base test class for convenience.

3. MistralException - added default message to enable default instantiation - make mock.assertRaise fails when exception has no default constructor

Change-Id: I708637e3995597a819571a90f3292d6e34deab4e
Merge "Add negative tests to api"