git-p4: show Perforce error to the user

git-p4: show Perforce error to the user

During "git p4 clone" if p4 process returns an error from the server,
it will store the message in the 'err' variable. Then it will send a
text command "die-now" to git-fast-import. However, git-fast-import
raises an exception: "fatal: Unsupported command: die-now" and err is
never displayed. This patch ensures that err is shown to the end user.

Signed-off-by: Fahad Alrashed <fahad@keylock.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>