Notify: add arming failure tone

libraries/AP_Notify/AP_Notify.h

@@ -42,6 +42,7 @@ public:
         uint16_t failsafe_radio     : 1;    // 1 if radio failsafe
         uint16_t failsafe_battery   : 1;    // 1 if battery failsafe
         uint16_t failsafe_gps       : 1;    // 1 if gps failsafe
+        uint16_t arming_failed      : 1;    // 1 if copter failed to arm after user input
 
         // additional flags
         uint16_t external_leds      : 1;    // 1 if external LEDs are enabled (normally only used for copter)

libraries/AP_Notify/ToneAlarm_PX4.cpp

@@ -64,6 +64,13 @@ void ToneAlarm_PX4::update()
     if (_tonealarm_fd == -1) {
         return;
     }
+    
+    if(flags.arming_failed != AP_Notify::flags.arming_failed) {
+        flags.arming_failed = AP_Notify::flags.arming_failed;
+        if(flags.arming_failed) {
+            play_tune(TONE_ARMING_FAILURE_TUNE);
+        }
+    }
 
     // check if arming status has changed
     if (flags.armed != AP_Notify::flags.armed) {

libraries/AP_Notify/ToneAlarm_PX4.h

@@ -41,6 +41,7 @@ private:
         uint8_t failsafe_battery   : 1;    // 1 if battery failsafe
         uint8_t gps_glitching      : 1;    // 1 if gps position is not good
         uint8_t failsafe_gps       : 1;    // 1 if gps failsafe
+        uint8_t arming_failed      : 1;    // 0 = failing checks, 1 = passed
     } flags;
 };