transport-helper: report errors properly

transport-helper: report errors properly

If a push fails because the remote-helper died (with fast-export),
the user may not see any error message. We do correctly die with a
failed exit code, as we notice that the helper has died while
reading back the ref status from the helper. However, we don't print
any message. This is OK if the helper itself printed a useful error
message, but we cannot count on that; let's let the user know that
the helper failed.

In the long run, it may make more sense to propagate the error back
up to push, so that it can present the usual status table and give a
nicer message. But this is a much simpler fix that can help
immediately.

While we're adding tests, let's also confirm that the remote-helper
dying is also detected when importing refs. We currently do so
robustly when the helper uses the "done" feature (and that is what
we test).  We cannot do so reliably when the helper does not use the
"done" feature, but it is not even worth testing; the right solution
is for the helper to start using "done".

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>