Debug mode is enabled by default and it will show the server side failure reason. When going in production mode, you should disable the debug mode by following config.
$g = new jqgrid(); $g->debug = 0;
If you wish to change the SQL errors, you can turn them off using following setting.
$g->error_msg="Some issue occured";
For custom message at server side data validation, refer demos/editing/server-validation.php