git-p4: add P4CommandException to report errors talking to Perforce

git-p4: add P4CommandException to report errors talking to Perforce

Currently when there is a P4 error, git-p4 calls die() which just exits.

This then leaves the git-fast-import process still running, and can even
leave p4 itself still running.

As a result, git-p4 fails to exit cleanly. This is a particular problem
for people running the unit tests in regression.

Use this exception to report errors upwards, cleaning up as the error
propagates.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>