Make Errors more useful

[letmutx]

Goal: To be able to pin-point a location where the malformed input is/error occurs.

Currently the Error type doesn't contain information on where an error happened. I think it'd be helpful if we have at least these details to start with:

• Class name/ClassId - the error occurred while reading this Class from the input.
• Method/Field - if relevant.
• Id's of items (say in string_ids, type_ids section etc.)