Error message if index column not found

[YohDeadfall]

Unfortunately, the crate uses proc_macro and unwrap in code. It causes really unhelpful messages issues by the compiler as it gives zero context where None was instead of an expected Option::Some.

Ideally, the crate should use proc_macro_error2 or any other fine crate for that. It requires a lot of efforts, so let it be just panic with something meaningful.

[Handy-caT]

@yoh, instead of proc_macro_error2 I'm using syn::Error's for proper error handling in codegen. #106