Text from http://tools.ietf.org/html/draft-ietf-oauth-v2-16. The value in the "error" parameter is a code from the following list:
•invalid_request: The request is missing a required parameter, includes an unsupported parameter or parameter value, or is otherwise malformed.
•unauthorized_client: The client is not authorized to request an authorization code using this method.
•access_denied: The resource owner or authorization server denied the request.
•unsupported_response_type: The authorization server does not support obtaining an authorization code using this method.
•invalid_scope: The requested scope is invalid, unknown, or malformed.
•4xx or 5xx HTTP status code (except for 400 and 401): The authorization server may set the "error" parameter value to a numerical HTTP status code from the 4xx or 5xx range, with the exception of the 400 (Bad Request) and 401 (Unauthorized) status codes. For example, if the service is temporarily unavailable, the authorization server may return an error response with "error" set to "503".