Refactor dialog transition return type

[adamchol]

Small simplification which makes rejecting calls return unit instead of Result<()>.

I noticed that DialogInner::transition() doesn't actually return Err(_) ever, so I just changed the return type to unit.

There might be more cases like that in the codebase.

[yeoleobun]

Yes, there are lot of things like this. Some are pattern, some inner Error are removed or just logged.
But if it is not a bug, like some Error should report, but actually not returned from inner, we intend to clean it up for a major version.