verify_pack: do not ignore return value of verification function

In verify_pack, a caller-supplied verification function is called. The function returns an int. If that return value is non-zero, verify_pack should fail. The only caller of verify_pack is in builtin/fsck.c, whose verify_fn returns a meaningful error code (which was then ignored). Now, fsck might return a different error code (with more detail). This would happen in the unlikely event that a commit or tree that is a valid git object but not a valid instance of its type gets into a pack. Signed-off-by: David Turner <dturner@twopensource.com> Signed-off-by: Jeff King <peff@peff.net>