Check for remote server exec failures and report

If remote.name.uploadpack or .receivepack is misconfigured and points
to a non-existent command on the remote system, we should receive back
exit status 127.  Report this case specially with the command we used
so the user knows what is going.

Bug: 293703
Change-Id: I7504e7b6238d5d8e698d37db7411c4817a039d08
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>