Check that pdf output file exists in genealogy tree reports

[Nick-Hall]

Fixes #10696.

[ennoborg]

We may get more information by processing the return value of the communicate call, like this:

outs, errs = proc.communicate()

The variables shown here store the stdout and stderr provided by the process, and may provide more information when the process runs out of memory, or when an incomplete install leads to missing styles.

[Nick-Hall]

The process outputs a huge amount of information.

For debugging, it is best to create a tex file and then run lualatex manually, then you can examine the log files.

[hgohel]

The current change isn't enough to fully fix 10696. By changing the call to Popen() in Windows to match the other platforms, report generation works on Windows.