Doesn't add much logic, only takes advantage of parser structure for the ordering enforcement.
Note import_nonexistent tests should probably fail, but writing import tests needs a chain of functionality, and I figured I'd just start adding some to validate the syntax (not adding existent imports because that'd require multi-file structure).
This is intended to:
- Standardize use of line_num (and highlight where no line_num is available).
- Standardize indications of runtime/compilation errors.
- Make it incrementally easier to write new errors.
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>