oak-run console export should handle exceptions such as missing segments

Problem: When trying to look at data using "pn" or running "export" oak-run console currently will choke and abort the traversal on exceptions such as SegmentNotFoundException.

Expected: It would be nice if it would be best-effort, log the issue or embed in the exported json, and just continue. This also ensures all missing segments for a subtree are listed with one command (if it fails right now you just see the first missing segment).