Handle /?error=...

[thekid]

This occurs when authentication fails - some providers decide not to shown an error page on their own but to redirect back to the original page and pass on an error response:

If the request fails due to a missing, invalid, or mismatching redirection URI, or if the client identifier is missing or invalid, the authorization server SHOULD inform the resource owner of the error and MUST NOT automatically redirect the user-agent to the invalid redirection URI.

See https://tools.ietf.org/html/rfc6749#section-4.1.2.1