Generate and send auth response when auth failed

[bochaco]

When the authorisation request is malformed, e.g. if it's requesting permissions for an invalid container name, we get the Err(NoSuchContainer("public")) error from the safe_authenticator library but it's not notifying neither the user nor the app which sent the request. It should at least send a response to the app encoding the error so it can also notify its user.

This is analogous to browser issue maidsafe/sn_browser#697