Raise an error if a remote context can't be retrieved due to cross-origin restrictions

[lanthaler]

Some sites (most notably purl.org) still don't set the CORS headers to allow cross-origin requests. The playground silently fails for documents using remote contexts that can't be accessed. It should show an error message explaining that the remote context couldn't be accessed.

/cc @dlongley @ivikash

[dlongley]

I'll look into this when I can. It used to throw an error for this ... so I don't know what's changed. If you have time to put a specific test url in here that would be helpful.

[dlongley]

Fixed here: digitalbazaar/jsonld.js@a62581a and ported over here: 2573021

[lanthaler]

Thanks a lot for fixing this so quickly Dave