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.