Add an Error variant for rust error

[Sytten]

Right now the Error enum is pretty much tied to the runtime, but sometimes we do need to throw stuff from the rust side other than IoError. Unsure what it would look like but Error::Custom(Box<dyn std::error::Error + Send + Sync>) is usually the most basic thing.

[Sytten]

Nvm I found the ctx.throw, I am pretty sure this is was the way it was designed