Make HSSNotFound a subclass of HTTPError(404) so that it does the right thing.
Add missing self parameter
Return None to trigger a 404 error without an exception; log when a call to an unknown API is made
Merge branch 'dev' into 404-on-hss-failure
Update UTs
Return 403 for unknown APIs to distinguish 'incorrect URL' from 'subscriber not found'
Fix typo and remove unused import
Clarify UTs by using default values and keyword arguments
Settle on 404 with 'Invalid API' reason in the body - couldn't figure out how to change the Reason phrase
Code review feedback - docstrings and matchers
Merge pull request #93 from Metaswitch/404-on-hss-failure

[Reviewer: Alex] Return 404 on HSS failure