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.