git-p4: better error reporting when p4 fails

git-p4: better error reporting when p4 fails

Currently when p4 fails to run, git-p4 just crashes with an obscure
error message.

For example, if the P4 ticket has expired, you get:

  Error: Cannot locate perforce checkout of <path> in client view

This change checks whether git-p4 can talk to the Perforce server when
the first P4 operation is attempted, and tries to print a meaningful
error message if it fails.

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