fix: narrow exception handling to json.JSONDecodeError with rich error details

Address PR #1892 review feedback from @aaunario-keeper:

- All json.loads sites: catch only json.JSONDecodeError (drop dead
  ValueError/TypeError/KeyError catches)
- Use e.msg, e.lineno, e.colno for actionable log messages
- import_schedule_field: split into separate json.JSONDecodeError and
  (ValueError, KeyError) handlers to distinguish parse vs structural errors

Author: jlima8900