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
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- Regex: '^"'
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