fix: return read_index error immediately

In some states `raft-rs` ignores the ReadIndex request.
Check it preliminary, don't wait for the timeout.

See for details:
- <https://github.com/tikv/raft-rs/blob/v0.6.0/src/raft.rs#L2058>
- <https://github.com/tikv/raft-rs/blob/v0.6.0/src/raft.rs#L2323>