YATT::Lite::Util::safe_render - To avoid error when $con != Connection

When widgets are called from YATT::Lite::Util::captured {...} block,
$con can be a plain IO::Handle, instead of (subclass of)YATT::Lite::Connection.
So, calling $con->error can cause error.

To avoid this, use con_error($con, $err, @params).