ENH: VPAW Visualize should show what is available, without an error message

The `VPAW Visualize` module should show whatever is available rather than giving an error message when components are missing. In particular, a `vpaw-root-dir` with the usual inputs `images/*` and `landmarks/*`but no pipeline outputs should not give an error message.

Start by locating the VPAW Visualize entry point and the handling of a vpaw-root-dir. Reproduce the case with images/* and landmarks/* but no pipeline outputs; done means the available inputs are shown without an error message.

Written by the indexing model from the issue text.