Improve arming of the drone in Autopilot

Improved logging.

Type: Improvement
Issue: https://denyspopov.atlassian.net/browse/AZ-21

src/az_drone_controller.cpp

@@ -136,8 +136,15 @@ bool AzDroneController::stateReadyForArming(void)
 
 bool AzDroneController::stateArm(void)
 {
-    // TODO!! Check return value and print warnings and errors.
-    return mAction->arm() == Action::Result::Success;
+    Action::Result result = mAction->arm();
+
+    if (result == Action::Result::Success) {
+        return true;
+    }
+    else {
+        std::cerr << "MAVSDK::Action::arm() failed. Reason: " << result << endl;
+        return false;
+    }
 }
 
 bool AzDroneController::stateTakeoff(void)