Improve use of MAVSDK::add_any_connection()

Added error printing and possibility to use UART as connection
to the flight controller.

Issue: https://denyspopov.atlassian.net/browse/AZ-16
Type: New Feature

src/main.cpp

@@ -9,8 +9,9 @@ int main(int argc, char *argv[])
     // This is needed to have main event loop and signal-slot events in the AzDroneController.
     QCoreApplication a(argc, argv);
 
-    if (argc != 2) {
-        qCritical() << "Please give mission JSON file as an argument.\n";
+    if (argc < 2) {
+        qCritical() << "\nPass the mission JSON file as the first argument.";
+        qCritical() << "A serial port connection can be enabled with \"serial\" as the second argument.\n";
         return 1;
     }