auth_radius: Improve error message to simplify debugging

(cherry picked from commit 6e51019b0a2c60178b9ca03b9bb2a5fef49ca7ba)
(cherry picked from commit c81c360315105e47f5fdd762ffb6490691c3c675)

modules/auth_radius/sterman.c

@@ -409,7 +409,7 @@ int radius_authorize_sterman(struct sip_msg* _msg, dig_cred_t* _cred, str* _meth
                         goto err;
                 }
 #endif 
-		LM_ERR("authorization failed\n");
+		LM_ERR("authorization failed. RC auth returned %d\n", i);
 	}
 
  err: