rhg: add error message for paths outside the repository when cwd != root This mirrors the Python implementation. The relative path handling should probably be refactored into a util, but it it out of scope for this change.

rust: don't swallow valuable error information This helps when diagnosing corruption, and is in general good practice. The information is here, valuable and can be used easily.

rust: add message to `DirstateV2ParseError` to give some context This is useful when debugging.

py3: stop using deprecated Element.getchildren() method in convert/darcs This has been deprecated since py3.2, and removed entirely in py3.9