Revisit behavior and clarify documentation around SourceNamespace

This turned out to be the root cause behind https://community.intersystems.com/post/errors-when-using-testcoverage-error-5002-objectscript-error . It would be nice if we didn't need SourceNamespace and everything could "just work" regardless of where the code lives. Alternatively, if it's needed, we should provide a more descriptive error message.

Should also investigate behavior of saving results when SourceNamespace is used, as this seemed off.