Prevent M420 error from disconnecting OctoPrint

Just echo the error instead of labeling it `Error:`.

Marlin/src/gcode/bedlevel/M420.cpp

 
     }
     else if (to_enable || seenV) {
-      SERIAL_ERROR_MSG("Invalid mesh.");
+      SERIAL_ECHO_MSG("Invalid mesh.");
       goto EXIT_M420;
     }