print full error message when non-api error (#474)

when type asserting fails here, err is reassigned with nil and the
default block of the switch prints out <nil> in the error message. This
makes debugging a configuration or access token issue difficult

The particular error this surfaces is:

Response: {
  "error": "unauthorized_client",
  "error_description": "Client is unauthorized to retrieve access tokens using this method, or client not authorized for any of the scopes requested."
}

Signed-off-by: Josh Bielick <jbielick@gmail.com>