Fix bug in handling of failed github authentication request

- The error scenario from the flask.oauthlib.authorized_response() doesn't
  return None.  It can return an Exception object as well.  So, checking for
  None is not very effective.
- This fixes the limited issue we've seen in production related to the return
  from the oauthlib not having the `access_token` in it.  This bug was leading
  to a `KeyError` and masking the real reason github didn't return us the
  token.

pskb_website/views.py

@@ -138,9 +138,9 @@ def authorized():
     """URL for Github auth callback"""
 
     resp = remote.github.authorized_response()
-    if resp is None:
-        flash('Access denied: reason=%s error=%s' % (
-              request.args['error'], request.args['error_description']),
+    if resp is None or resp.get('access_token') is None:
+        flash('Access denied: reason=%s error=%s resp=%s' % (
+              request.args['error'], request.args['error_description'], resp),
               category='error')
         return redirect(url_for('index'))