Revisit behavior and clarify documentation around SourceNamespace

[isc-tleavitt]

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.

[isc-tleavitt]

Actually seems like the issue was related to having bad things cached; running ##class(TestCoverage.Utils).Clear() fixed the issue.
I think we should always treat the cache as invalid if ExecutableLines shows nothing could be run.