verify_pack: do not ignore return value of verification function

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 <redacted>
Signed-off-by: Jeff King <redacted>