Fixed `postLoginR`.

postLoginR now uses `InvalidUsernamePass` from `Yesod.Auth.Message` instead of a default english message.

I have not fixed `getAuthIdHashDB` since it does not have the right signature, at least I can't find how to fit it in.
Incremented `defaultStrength` to `15`.
Changed `getAuthIdHashDB`'s error message to something less specific.