Make errors handleable

Most of errors that caused by unexpected behavior of the other side's RTO port will just lead to panic.
However, especially in Foundry context, the other port might have been implemented by 3rd party and could be unreliable.
In this kind of cases, user will just want to handle the error, not panic.

One trivial solution is adding a `Failed` state for the RTO context. The context will be on `Failed` state when it encounters unexpected protocol error, and will be stopped. Users can notice this, and can handle with theirs means.

One drawback of supporting such error handling is the interface of service object; it might be mandatory to all methods to have `Result<>` as return types.

Depends on #76
Depends on #83