error: introduce box.info.ro_reason

Follow-up #5568

@TarantoolBot document
Title: box.info.ro_reason

The new `box.info` field - `ro_reason` - is `nil` on a writable
instance. On a read-only instance `box.info.ro == true` it reports
an error reason. Currently the list is
- `'election'` - `box.cfg.election_mode` is not `'off'` and this
  instance is not a leader. See `box.info.election` for details.
- `'synchro'` - the synchro queue is owned by some other instance.
  For details see `box.info.synchro`.
- `'config'` - `box.cfg.read_only` is true;
- `'orphan'` - the instance is in orphan state.