Add exceptions for OpenStack clients

These exceptions can be used in novaclient, keystoneclient,
glanceclient, and other client projects.

Concept of this file comes from cloudservers/exceptions.py
and novaclient/exceptions.py. HTTP exceptions were rewritten.

Partially implements: blueprint common-client-library

Change-Id: I9e17d855f7c26284ce66b13515aa4a0b8855347a
Merge "Add exceptions for OpenStack clients"