Better error messages on failed oauth call

[pajlada]

we should print whether or not the error was caused by bad redirect uri or not

[RAnders00]

On error, the user is sent to <REDIRECT_URI>?error=error_code&error_description=A+readable+description

Note for correctness: The OAuth standard only mandates the ?error=error_code parameter to be present, and Twitch doesn't specify anywhere that they send error_description all the time. We should display the error description if it's present, but should gracefully handle the case when it's not present. https://tools.ietf.org/html/rfc6749#section-4.1.2.1