Add unit tests for Restlib.validate_response
Ignore json errors in validate_response

Handle everything except 301, which is
still undecided.
998033: Handle 401 and 403 with no response body

Merge python-rhsm/pull/91 and python-rhsm/pull/79.

Add UnauthorizedException and ForbiddenExceptions,
and raise them on 401 and 403 responses that have
no valid JSON body. These classes are subclasses
of RemoteServerException so client code should
work without changes.

Also handle responses with no 'content' item in
the response body.
Merge pull request #92 from candlepin/alikins/998033_validate_response_update

Alikins/998033 validate response update