Unified exceptions and better exception handling

[analogue]

Fixes #153

bravado.exception.HTTPError is raised consistently by both Requests and Fido clients.
bravado_core.exception.SwaggerMappingError is raised during swagger outgoing request processing or swagger incoming response processing failures.

The exception story:

• When there is no response_callback (just a plain old http req/resp) then HTTPError is raised
• When making a service call that fails with a 5XX, then HTTPError is raised with minimal info
• When making a service call that succeeds, no exception is raised
• When making a service call that has a non-2XX response that matches a response in the spec, a HTTPError is raised with the swagger_result noted in the spec
• When making a service call that has a non-2XX response that doesn't match any responses in the spec, then HTTPError is raised without a swagger_result

Depends on Yelp/bravado-core#39

[bpicolo]

Lg2m : ).

travis failing expecting a bravado-core version that's not on pypi:
No matching distribution found for bravado-core>=2.3.0 (from bravado==3.0.0)

[dnephin]

lgtm once travis is green