If an --author argument is specified but does not contain a '>' then git tries
to find the argument within the existing authors; and gives the error
message "No existing author found with '%s'" if there is no match.

This is confusing for users who try to specify a valid complete author
name.

Rename the error message to make it clearer that the failure has two
reasons in this case.

(This codepath is touched only when we know already that the argument
cannot be a completely wellformed author ident.)
Signed-off-by: NMichael J Gruber <git@drmicha.warpmail.net>
Helped-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>