Create Base class for Mistral Exceptions and Errors
Create Base class for Mistral Exceptions and Errors

Moved common logic in mistral/exceptions from
MistralError and MistralExceptions to MistralFailureBase

Change-Id: I79dc7b91c00a296a2c6d9c3b589ea3fa4fc7db48

openstack/mistral,openstack/mistral