Fix Clippy Warnings + Error Handling

[rolandpeelen]

Some basic refactors. Most of it was automatic. Biggest (manual) changes:

• In parse.rs we now hoist module.source_type.to_owned() to the root match statement, so we don't have to do it in every sub-tree. This makes the code quite a lot more readable
• We stop returning unit for the return of build and have some basic error Enums.