Augment TokenEndpointClientService's error codes with descriptions from the specs

[leonardochaia]

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

[leonardochaia]

Also consider that the endpoint could optionally return that:

error_description
         OPTIONAL.  Human-readable ASCII [USASCII] text providing
         additional information, used to assist the client developer in
         understanding the error that occurred.
         Values for the "error_description" parameter MUST NOT include
         characters outside the set %x20-21 / %x23-5B / %x5D-7E.

   error_uri
         OPTIONAL.  A URI identifying a human-readable web page with
         information about the error, used to provide the client
         developer with additional information about the error.
         Values for the "error_uri" parameter MUST conform to the
         URI-reference syntax and thus MUST NOT include characters
         outside the set %x21 / %x23-5B / %x5D-7E.