git-p4: reinterpret confusing p4 message

git-p4: reinterpret confusing p4 message

Error output will look like this:

glom$ git p4 clone //deopt
Importing from //deopt into .
Reinitialized existing Git repository in /tmp/x/.git/
Doing initial import of //deopt from revision #head into refs/remotes/p4/master
p4 returned an error: //deopt/... - must refer to client glom.

This particular p4 error is misleading.
Perhaps the depot path was misspelled.
Depot path:  //deopt

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>