Cleaned up exception module and added unittests

Removed unused exceptions and fixed several inheritance
issues, that causes wrong responce codes.

exceptions MigrationNotFound and MigrationNotFoundByStatus
are left, because manila just doesn't have migrations yet.

Change-Id: Icf39422927fc7754c5c47ab717fad178ed44a0dc
Related-Bug: #1325934
Merge "Cleaned up exception module and added unittests"