If you provide a bad wkhtmltopdf option, you should get an exception

If you provide an invalid wkhtmltopdf option and wkhtmltopdf fails to
execute, you should be told about this straight away, by an exception
being raised.

We had this problem where we were using an invalid argument to wkhtmltopdf, as well as ``'quiet': None``, so we weren't seeing the error and had to dig through.

Without this patch, pdfkit will happily just return an empty string as if the PDF generation was working fine.

I've included tests.