Save some RAM in MBL's G29

Marlin/Marlin_main.cpp

           }
         }
         else {
-          SERIAL_PROTOCOLLNPGM("X not entered.");
+          SERIAL_CHAR('X'); SERIAL_PROTOCOLLNPGM(" not entered.");
           return;
         }
+
         if (code_seen('Y')) {
           py = code_value_int() - 1;
           if (py < 0 || py >= MESH_NUM_Y_POINTS) {
           }
         }
         else {
-          SERIAL_PROTOCOLLNPGM("Y not entered.");
+          SERIAL_CHAR('Y'); SERIAL_PROTOCOLLNPGM(" not entered.");
           return;
         }
+
         if (code_seen('Z')) {
           mbl.z_values[py][px] = code_value_axis_units(Z_AXIS);
         }
         else {
-          SERIAL_PROTOCOLLNPGM("Z not entered.");
+          SERIAL_CHAR('Z'); SERIAL_PROTOCOLLNPGM(" not entered.");
           return;
         }
         break;
           mbl.z_offset = code_value_axis_units(Z_AXIS);
         }
         else {
-          SERIAL_PROTOCOLLNPGM("Z not entered.");
+          SERIAL_CHAR('Z'); SERIAL_PROTOCOLLNPGM(" not entered.");
           return;
         }
         break;