Support Clone on error types

[Bajix]

For batching queries it is necessary to be able to clone errors across all receivers. Perhaps we could add support for this by using a library that supports string cloning, such as flexstr or arcstr

[weiznich]

I fear that's not really possible because diesel uses boxed errors in quite a few places (Box<dyn std::error::Error>). We do not even control the inner error type in some cases. Introducing an additional Clone bound there would be a huge breaking change. Additionally it seems like clonable error types is generally nothing that is often used in the rust ecosystem.