Improve error reporting

[dlongley]

CBOR-LD is designed such that it expects valid JSON-LD to be passed to it for compression, however, we should have some better error messages both during compression and decompression when we encounter unexpected types, etc.

[gatemezing]

Hi @dlongley . Yes, that's definitively very useful. We are trying to make a small Web App to use this awesome library but we are not sure how to handle for e.g., conneg on URIs of context.