Shorter M32 error message

Marlin/src/sd/cardreader.cpp

 
       // Too deep? The firmware has to bail.
       if (file_subcall_ctr > SD_PROCEDURE_DEPTH - 1) {
-        SERIAL_ERROR_MSG("trying to call sub-gcode files with too many levels. MAX level is:" STRINGIFY(SD_PROCEDURE_DEPTH));
+        SERIAL_ERROR_MSG("Exceeded max SUBROUTINE depth:" STRINGIFY(SD_PROCEDURE_DEPTH));
         kill();
         return;
       }