git-clone: tell the user a bit more about clone-pack failure.

git-clone: tell the user a bit more about clone-pack failure.

When clone-pack has trouble with the remote, it dies unfriendly
"EOF" error message.  We cannot tell the reason why it failed
from the local end; it could be that the repository did not
exist, or configured not to serve over git-daemon, or a network
failure.  At least, saying clone-pack failed makes it a bit more
meaningful.

I am not convinced yet that removing the newly created directory
is the right thing to do, so this commit leaves the new
directory behind.

Reported by Sam Ravnborg.

Signed-off-by: Junio C Hamano <junkio@cox.net>