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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}