Spring cleaning: error log

[cristianoc]

The error log is difficult to parse for tooling. The vscode extension routinely has bugs parsing errors printed in rarely used corners of the compiler codebase.
It would be better to emit structured information in lib/bs/.compiler.log in json.
Of course, changing all the errors at once would be a big task.
This can be done gradually, by providing json output in lib/bs/.compiler.log for certain errors. This is how it could work:

1. design minimalistic json format for errors, with a printing functionality
2. convert at least a couple of existing error messages to output this new format, and make sure the printing functionality preserves the errors printed (or improves them)
3. have the vscode extension parse this new json format
4. gradually convert more and more cases of error messages to the new format
5. eventually make json the only format used, and delete the complex error parsing logic in the extension

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[zth]

Cc @mununki