Copter: fix nack result_unsupported when arming fails

We should return unsupported if param1 does not equal 0 or 1

ArduCopter/GCS_Mavlink.pde

@@ -1208,6 +1208,8 @@ void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)
             } else if (packet.param1 == 0.0f && (mode_has_manual_throttle(control_mode) || ap.land_complete))  {
                 init_disarm_motors();
                 result = MAV_RESULT_ACCEPTED;
+            } else {
+                result = MAV_RESULT_UNSUPPORTED;
             }
             break;