Print an error on unknown D-codes

This follows the same convention of M/G codes, so that the user knowns
that the D-code has been either handled or ignored.

Firmware/Marlin_main.cpp

@@ -9521,6 +9521,9 @@ Sigma_Exit:
 #endif //FILAMENT_SENSOR
 
 #endif //DEBUG_DCODES
+
+    default:
+        printf_P(PSTR("Unknown D code: %s\n"), cmdbuffer + bufindr + CMDHDRSIZE);
 	}
   }