PX4 Firmware plugin: First try to switch flight mode, then arm if successful - which is a lot safer than arming and then seeing if we can switch modes.

src/FirmwarePlugin/PX4/PX4FirmwarePlugin.cc

@@ -452,12 +452,15 @@ void PX4FirmwarePlugin::guidedModeChangeAltitude(Vehicle* vehicle, double altitu
 
 void PX4FirmwarePlugin::startMission(Vehicle* vehicle)
 {
-    if (!_armVehicleAndValidate(vehicle)) {
-        qgcApp()->showMessage(tr("Unable to start mission: Vehicle failed to arm."));
-        return;
-    }
 
-    _setFlightModeAndValidate(vehicle, missionFlightMode());
+    if (_setFlightModeAndValidate(vehicle, missionFlightMode())) {
+        if (!_armVehicleAndValidate(vehicle)) {
+            qgcApp()->showMessage(tr("Unable to start mission: Vehicle rejected arming."));
+            return;
+        }
+    } else {
+        qgcApp()->showMessage(tr("Unable to start mission: Vehicle not ready."));
+    }
 }
 
 void PX4FirmwarePlugin::setGuidedMode(Vehicle* vehicle, bool guidedMode)