if there's a problem logging in, return to the login page

@aaronlifshin This is the error handling I was telling you about, in case the user cancels the authorization. Easiest to demonstrate by selecting to login with Twitter, cancelling the authorization, then clicking on "Return to mayday-us"
It's a tiny change, but I didn't want to push it moments before release and screw anything up. So I'll leave this  PR for you to merge in if you want to deploy it.