Improve unit-description generator's OCI appVersion failure handling: show actionable error traces and avoid misleading N/A fallback

When generating units documentation, the OCI chart appVersion lookup failures are silently swallowed (for example Docker Hub rate limits and others ).

The generator then writes App-version: N/A like:

which looks like a real chart metadata value instead of a retrieval failure.

cc @alexandru.ghita.orange

cc @marc.bailly1 @satyawanj