cborld command doesn't handle 404 context URLs

[davidlehn]

Trying to encode a file with a 404 context URL can result in parse errors on whatever the 404 content was.

[dlongley]

All issues related to the CLI should be moved to a new cborld-cli repo. See digitalbazaar/cborld#35. If we want to just disable the CLI in the 4.0 release and remove it entirely from the README, that would be my vote.