Raft::new() returns a Result

[Hoverbear]

Depends on:

• Insert Learner/Voter should not panic. #88

Currently calling Raft::new() will panic if it is provided an invalid configuration.

https://github.com/pingcap/raft-rs/blob/18f1cd8bb1a452dd9a2e3a8c0b507fc138c1862d/src/raft.rs#L390-L391

The consequence of this is that the user of the Raft is not provided an opportunity to handle certain types of error.

Fixing this issue would include changing Raft::new() to return a Result<Raft<T>> instead of a Raft<T>. In order to do this you may need to add new error variants:

https://github.com/pingcap/raft-rs/blob/18f1cd8bb1a452dd9a2e3a8c0b507fc138c1862d/src/errors.rs#L19-L22.