Rename HTTPError -> HttpError

With the move to the apiclient exceptions from oslo the basic HTTP error
class was renamed. This was not reflected in all places in the code.

It was also not picked up by the tests because the apiclient tests
weren't running due to a missing __init__.py file. Because this should
be backwards compatible it was added to the list in exceptions, the
check that this is available is in the (now running) apiclient tests.

Blueprint: common-client-library-2
Change-Id: I307c1083f29e3207cc86aa938043270e5c32b4bb
Merge "Rename HTTPError -> HttpError"