Resolve error message from OAuth error

[typerandom]

When a Facebook OAuth callback errors then resolve the error message from Facebook's error structure.

How to verify

1. Start an example application and navigate to /login.
2. Click the login to Facebook login link.
3. While being on the Facebook OAuth provider page prompting you to authorize your application to access your Facebook account. Go into the Facebook Developer dashboard and remove your whitelisted callback URI.
4. Go back to the Facebook provider page and authorize your application to access the account.
5. You should be redirected back to /login and with an error message stating [object Object].
6. Redo steps 1-4 but without removing your whitelisted callback URI.
7. You should now see a proper error message (not [object Object]).

Fixes #528

[coveralls]

Coverage decreased (-1.6%) to 69.284% when pulling 87e82a9 on fix-facebook-oauth-error-message into 5b4d8bd on master.