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

Moved from GitHub ratel/221

Posted by paulftw:

See https://github.com/dgraph-io/ratel/issues/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.