/complete/<backend> crashing

Facebook redirects users to /complete/facebook when they click on shared via APP_NAME in posts that have beed shared via FB app. This is annoying since the purpose of that url is for facebook callbacks to complete user authentication. Since we can't specify two different URLs in FB App settings it makes sense to fix the bug that causes the page to crash if complete url is called with no completion parameters.

This commit fixes the HTTP 500 error that was generated if no parameters were provided (as they are if users login via /login/facebook). It now redirects user to error page if configured.