Improve how we type error messages in the compiler

[infomiho]

Based on this comment: #2297 (comment)

We often return some String errors and to give more context we create a type alias e.g. type ErrorMsg = String or type CompileError = String. While this makes sense, we'll keep adding these aliases to String and we'll have a lot of similarly named aliases.

Maybe we shouldn't alias String and use a proper data type for errors?