Denying access to the Facebook/Google elsewhere throws a 500

[ghost]

This behaviour was originally reported by @apple1337 on H1-160418.

Actual behaviour

• Go on https://gratipay.com,
• Select the Facebook elsewhere,
• (if applicable) Login on Facebook,
• Deny the Gratipay request to access account information.
• Get redirected to a page showing one error 500.

Expected behaviour

The user should be redirect to one generic failure page, depending of the information we get from Facebook.