AP_Scripting: add arming check for failed scripts

libraries/AP_Scripting/lua_scripts.h

@@ -141,4 +141,7 @@ public:
     // must be static for use in atpanic, public to allow bindings to issue none fatal warnings
     static void set_and_print_new_error_message(MAV_SEVERITY severity, const char *fmt, ...) FMT_PRINTF(2,3);
 
+    // return last error message, nullptr if none
+    static const char* get_last_error_message() { return error_msg_buf; }
+
 };