Never return errors on the wrong line

[hedning]

This will require walking through the stack trace and leaving errors in the correct files.

Adding errors can be done in two ways.

1. Add all the stacks as separate errors, which might be too noisy (at least with all the wrapper calls)..
2. Only add the deepest stack, potentially filtering out files not open or in the extension

Parsing the stack should be quite straightforward in elisp or js (though there's not that much to gain doing it in js I think).

[hedning]

I'm pretty sure this works OK now.