git p4: generate better error message for bad depot path

Depot paths must start with //.  Exit with a better explanation
when a bad depot path is supplied.

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

git-p4.py

@@ -3163,6 +3163,7 @@ class P4Clone(P4Sync):
         self.cloneExclude = ["/"+p for p in self.cloneExclude]
         for p in depotPaths:
             if not p.startswith("//"):
+                sys.stderr.write('Depot paths must start with "//": %s\n' % p)
                 return False
 
         if not self.cloneDestination: