lfs: provide more Batch API error info via a hint in the raised exception
A coworker had a typo in `lfs.url`, forgot it was even set because usually the
blob server is inferred, and then got a 404. It would have been easier to debug
with the failing URL printed.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}