crimson/common/errorator: fix errorator::pass_further and discard_all

crimson/common/errorator: fix errorator::pass_further and discard_all

Previously, both of these were invocable on all errors, but would
static_assert on invalid ones.  What we actually want is for them
to only be invocable on valid errors.  That way, we can do, for
instance:

  }).handle_error(
    roll_journal_segment_ertr::pass_further{},
    SegmentManager::open_ertr::all_same_way([this](auto &&e) {
      logger().error(
	"error {} in close segment {}",
	e,
	current_journal_segment_id);
      ceph_assert(0 == "error in close");
      return;
    })

to explicitely propogate any errors in roll_journal_segment_ertr
while asserting on anything else.

Signed-off-by: Samuel Just <sjust@redhat.com>