Support User defined Error Handling

[drmason13]

Currently none of the trait methods contain Result in their type signature, so ? won't work in their implementations.

This encourages a lot of unwrap/expect where I would rather discourage it.

I initially avoided using Result in the trait methods because I want to display the output, so I required Display. However Result doesn't implement Display. I think this might be solvable with a more complicated trait bound combining both Result and Display.

Also, that then forces everyone to return a Result in their implementations, that might be a good thing though since this library aims to be used by beginners to Rust.

This would be a breaking change so the sooner the better!