Add more checking to checker class to vet context entries, make sure they are actully JSON

[ptsefton]

What we are aiming for is:

• Running a check (before you load the crate) eg in the playground will show if there is a link to an RO-Crate context and ALSO vet any additional entries to make sure they actully are JSON objects that can be used a @context. Return useful errors like "https://w3id.org/ro/terms/earth-science#" resolves to a text file that is not JSON-LD and cannot be used as an entry in @context
• Loading an RO-Crate will not fail if there is rubbish in the @context - it should ignore stuff like references to HTML pages or CSV (both of which we have come across).