Add a visual hint when a mutation passes or fails JSON.parse

[paulftw]

See #220

In that issue Ratel is doing (sort of) the right thing, but if there's a hard to spot error in JSON users are getting confused to why it's not interpreted as JSON.

Full solution involves:

1. UI to show (and force?) selection of JSON/RDF
2. Syntax highlight of where JSON parser is failing

Step 2 should be covered by other tasks.

[minhaj-shakeel]

Github issues have been deprecated.
This issue has been moved to discuss. You can follow the conversation there and also subscribe to updates by changing your notification preferences.