Handling error_subcode field in error responses

Most of the OAuth errors also return an error_subcode field. Different subcodes require
different recovery tactic, as defined in Facebook's reference:
https://developers.facebook.com/docs/reference/api/errors/

The implementation breaks binary compatibility, as the signature of
FacebookExceptionMapper.exceptionForTypeAndMessage was changed.
Merge pull request #59 from egergo/error-subcode

Handling error_subcode field in error responses