Error messages

In case of errors, the OAuth 2.0 server will return a JSON response, containing an object with members “error” and “error_description”.

E.g.

{
  "error": "invalid_grant",
  "error_description": "Invalid username and password combination"
}

De OAuth 2.0 specification describes all possible errors:

https://tools.ietf.org/html/rfc6749#section-5.2