wkhtmltopdf: Improve error reporting
When wkhtmltopdf failed, there was no error reporting, apart from the
fact that the output file was missing.
This commits modernizes the call to wkhtmltopdf to use the new run
function available in Python 3, which supports a check parameter to
check the value returned by the program, and raise an exception if it is
not 0, including all relevant information.
In addition to that, any possible message that wkhtmltopdf outputs
in a successful execution is now logged.