fix!: derive Reply macro now uses kameo Infallible instead of ()

[meowjesty]

I believe the derive macro using type Error = () was not intended? I was getting some annoying errors saying that the error did not implement Display, and this was the cause.

std types implement Reply with kameo::error::Infallible, so this PR changes the derive Reply macro to do the same.

[tqwewe]

Looks like its something that was forgotten! I think previously I used () because kameo didn't have its own Infallible type, and was using std::convert::Infallible which doesn't impl Serialize/Deserialize.

I'll try this change out on some of my existing code base which uses kameo to see if there's any issues, but should be a nice improvement