Support an OAuth2 error response

[twistedstream]

The Playground works great when the authentication flow results in success (redirect back to https://openidconnect.net/callback with code and state). But when a failure occurs (redirect with error and error_description), the page hangs for a bit and then just crashes.

To reproduce, open this URL in the browser:
https://openidconnect.net/callback?error=access_denied&error_description=User%20did%20not%20authorize%20the%20request&state=ba21b24c5960f3f240302bf940f6c331a73fdd72

After several seconds, you'll see this page: