git-p4: better message for "git-p4 sync" when not cloned
git-p4: better message for "git-p4 sync" when not cloned

A common error is to do "git-p4 sync" in a repository that
was not initialized by "git-p4 clone".  There will be no
p4 refs.  The error message in this case is a traceback
for an assertion, which is confusing.

Change it instead to explain the likely problem.

Signed-off-by: Pete Wyckoff <1a91d62f7ca67399625a4368a6ab5d4a3baa6073@padd.com>
Acked-By: Tor Arvid Lund <10881b5c954573e37687444135aa1ca5b764260c@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git