tests: print the server error log in `test-url-download.t`
There was a stray 500 error in CI, but no additional context to know what the
issue was.
https://foss.heptapod.net/octobus/mercurial-devel/-/jobs/163093
Differential Revision: https://phab.mercurial-scm.org/D9949
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.