The Exception thrown by the package is an UnauthorizedException which you can see has a requiredRoles property. If you handle that exception in app/Exceptions/Handler.php, you can get the role from the Exception, and rethrow a new exception with your preferred message, e.g.
public function render(Throwable $e)
{
if ($e instanceof \Spatie\Permission\Exceptions\UnauthorizedException) {
throw new HttpException($e->getCode(), 'Access Denied');
}
parent::render($e);
}