Wkhtmltopdf – better error reporting

Hi,

When something is wrong with calling external binary to render PDF and the command gives no output on standard error it would be very helpful to give admin a hint what exactly command it was.

Could someone with repo write access change one line in print/print_pdf/lib_handlers/print_pdf_wkhtmltopdf/print_pdf_wkhtmltopdf.pages.inc

From:

$error = 'No stderr output available.';

To:

$error = 'No stderr output available. Command was: ' . $cmd;