crimson/os/seastore: expand error type for reads to include eagain

crimson/os/seastore: expand error type for reads to include eagain

There are cases where reads may detect an invalid cached extent and require
the user to restart the transaction.

As a side effect, clean up errorators to assert cases where construction
is invalid (invalid refcounts, enoent or invarg on node read, etc).  In
such cases, we may eventually want to introduce an internal_error error
type, but asserting is fine for now as we do not expect users to handle
these cases.

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