Handle OAuth Errors

[wbobeirne]

When OAuth fails, we get redirected to a page with query parameters that contain more information. Here's an example of a GitHub OAuth failure redirect:

/?error=redirect_uri_mismatch&error_description=The+redirect_uri+MUST+match+the+registered+callback+URL+for+this+application.&error_uri=https%3A%2F%2Fdeveloper.github.com%2Fapps%2Fmanaging-oauth-apps%2Ftroubleshooting-authorization-request-errors%2F%23redirect-uri-mismatch&state=azCYm71U9Kg8Fpv85HzaGB94uBNANw

We should handle these and display them to the user. Should also be useful for development.