Exception handling in magnum remote commands

Problem:
Currently whenever any remote commands raise an exception, that exception is returned as RemoteError and user receives 500 status which is not correct behaviour.

For eg.
On deleting a non existing bay, BayNotFound exception is raised and that exception is wrapped in RemoteError(exc_type=None, value=None, traceback=None). The oslo tries to deserialize this error by looking for exceptions defined in allow_remote_exmods and then re-raise them. But it failed to do so because magnum exceptions were not registered in allow_remote_exmods.

Proposed Solution:
All the remote exceptions must be deserialised to actual exceptions raised. Then re-raise the appropiate exception from controller to client.

This blueprint will do all the necessary changes to handle remote exceptions and their test cases.