Don't reject uploads with URLs as "empty" if the URL cannot be fetched

[gsnedders]

c.f. #4020 (comment) in combination with:

wpt.fyi/results-processor/processor.py

Lines 329 to 333 in 2b78c27

	p.download(results, screenshots, archives)
	if len(p.results) == 0:
	_log.error("No results successfully downloaded")
	p.update_status(run_id, 'EMPTY', None, callback_url)
	return ''

and:

wpt.fyi/results-processor/processor.py

Lines 157 to 159 in 2b78c27

	artifact = self._download_http(archive_url)
	if artifact is None:
	return

and:

wpt.fyi/results-processor/processor.py

Lines 139 to 141 in 2b78c27

	except requests.HTTPError:
	_log.error("Failed to fetch (%d): %s", r.status_code, url)
	return None

While it's technically true that the result set is empty, the fact that it's empty because we failed to download anything is distinctly different to actually receiving no results.