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>

git-p4

@@ -1855,6 +1855,10 @@ class P4Sync(Command):
             if 'code' in info and info['code'] == 'error':
                 sys.stderr.write("p4 returned an error: %s\n"
                                  % info['data'])
+                if info['data'].find("must refer to client") >= 0:
+                    sys.stderr.write("This particular p4 error is misleading.\n")
+                    sys.stderr.write("Perhaps the depot path was misspelled.\n");
+                    sys.stderr.write("Depot path:  %s\n" % " ".join(self.depotPaths))
                 sys.exit(1)
             if 'p4ExitCode' in info:
                 sys.stderr.write("p4 exitcode: %s\n" % info['p4ExitCode'])