UC-116 Facebook Client Service & improved user-mapping related error handling

This adds a Facebook Client Service to unify the way user mappings are created. This allows for all controller logic to go through the service in order to create a user mapping. This service is the front for communicating with FacebookMapModel (partially for now). We can then refactor more operations later into the service to avoid duplicate logic in multiple places.
This also simplifies error handling; now that we have more error cases to cover.

* Also excludes UserSignup special page from return-to logic, so that user does not land on it after logging in.

https://wikia-inc.atlassian.net/browse/UC-116
These public classes can be caught no matter where they're defined.  If we had a large number of rich exception classes, I would be more convinced we should have a dedicated folder to house these, and define *all* exceptions here.  However we're essentially just defining constants here; exception classes that just have a name that reminds us of the class they are thrown from.  Having this single parent exception in a separate folder is confusing IMO.