Make `ToGodot` and `FromGodot` conversions use `Result`

[lilizoey]

In #421 we created a unified api for converting types across ffi. However every ToGodot conversion was assumed infallible, and FromGodot uses Option<T> to signal fallibility. This was to limit the scope of that PR, but means that error reporting isn't as good as it could be. It likely also meant a small regression is error messages.

We should change these to use Result instead to be able to handle errors better and with better error messaging.