A warning message is unsatisfying. But erroring out is too hard a failure,
especially since it may well work fine if the user has enabled passwordless
ssh.
I did think about falling back to one ssh connection at a time in this
case, but it would have needed a rework of every ssh call, which
seems far overboard for such a niche problem. There's no single place where
git-annex runs ssh, so no one place that it could block a concurrent call
on a semaphore. And, even if it did fall back to one ssh connection at a
time, it seems to me that doing so without warning the user about the
problem just invites bug reports like "git-annex is ignoring my -J2 and
only doing one download at a time". So a warning is needed, and I suppose
is good enough.
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