Make diagnostic to github annotation conversion infallible

[saecki]

Instead, just omit the path and span information.

Fixes #63

[elegaanz]

I'm not sure the GitHub API will accept that, according to the docs annotations require start_line and end_line. Maybe we can instead fallback to line 1 for both?

[elegaanz]

Another option I have considered when the conversion fails is to embed the annotation as part of the check summary message, instead of tying it to a specific line.

[saecki]

Interesting, when using the ::error workflow command, these are optional: https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands#setting-an-error-message
We could check if the documentation is just wrong and otherwise switch to the workflow command interface for annotations?

This seems to be a difference between check run and workflow annotations 🤔