AP_Airspeed: Tag a uninit airspeed sensor as a failure when calibrating

This helps catch the case where a GCS commands a calibration, and the
command is reported as MAV_RESULT_ACCEPTED, even though the actual state
is that at least one sensor backend failed. We are still letting the
other sensors be calibrated in this situation as a GCS may not be paying
proper attention to what happens with the MAV_RESULT and a user could
miss the status text, so it's safer to calibrate everything we can, and
then report a failure afterwards.

libraries/AP_Airspeed/AP_Airspeed.cpp

@@ -526,7 +526,8 @@ void AP_Airspeed::calibrate(bool in_startup)
             continue;
         }
         if (sensor[i] == nullptr) {
-            GCS_SEND_TEXT(MAV_SEVERITY_ERROR, "Airspeed %u not initalized, cannot cal", i+1);
+            GCS_SEND_TEXT(MAV_SEVERITY_ERROR, "Airspeed %u not initialized, cannot calibrate", i+1);
+            calibration_state[i] = CalibrationState::FAILED;
             continue;
         }
         state[i].cal.start_ms = AP_HAL::millis();