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.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.