Print error for M42 bad pin

Marlin/Marlin_main.cpp

@@ -4230,7 +4230,11 @@ inline void gcode_M42() {
   if (pin_number < 0) return;
 
   for (uint8_t i = 0; i < COUNT(sensitive_pins); i++)
-    if (pin_number == sensitive_pins[i]) return;
+    if (pin_number == sensitive_pins[i]) {
+      SERIAL_ERROR_START;
+      SERIAL_ERRORLNPGM(MSG_ERR_PROTECTED_PIN);
+      return;
+    }
 
   pinMode(pin_number, OUTPUT);
   digitalWrite(pin_number, pin_status);

Marlin/language.h

@@ -151,6 +151,7 @@
 #define MSG_ERR_M421_PARAMETERS             "M421 requires XYZ or IJZ parameters"
 #define MSG_ERR_MESH_XY                     "Mesh XY or IJ cannot be resolved"
 #define MSG_ERR_ARC_ARGS                    "G2/G3 bad parameters"
+#define MSG_ERR_PROTECTED_PIN               "Protected Pin"
 #define MSG_ERR_M428_TOO_FAR                "Too far from reference point"
 #define MSG_ERR_M303_DISABLED               "PIDTEMP disabled"
 #define MSG_M119_REPORT                     "Reporting endstop status"