Handle FSM.Apply errors in Server.raftApply to avoid missing the error

[dnephin]

Almost everywhere we call Server.raftApply we have this same code to unpack an error out of the response:

if respErr, ok := resp.(error); ok {
    err = respErr
}

This is because FSM errors are reported from ApplyFuture.Response and only raft errors come from ApplyFuture.Error.

In some cases we completely ignore the response from the future, which would result in missing any errors returned by the FSM.Apply. Generally we want to log or return these errors so that the user knows something failed.

By handling the response error in Server.raftApply we can ensure that any error from the apply is always returned in the error response value, and we remove the need to check the response in every caller. We may be able to completely remove the response value, if the only use is unpacking this error.