AP_InternalError: add internal error for link errors in mission protocol

libraries/AP_InternalError/AP_InternalError.h

@@ -49,6 +49,7 @@ public:
         iomcu_fail                  = (1U << 13),
         spi_fail                    = (1U << 14),
         main_loop_stuck             = (1U << 15),
+        gcs_bad_missionprotocol_link= (1U << 16),
     };
 
     void error(const AP_InternalError::error_t error);