[FTS] improve JSON parsing error message

If you pass in the wrong type of information, for example, trying to pass an array in where an object is expected you get something like:

json: cannot unmarshal array into Go value of type map[string]interface {}

This doesn't make much sense to the end users. A better message would include which field was the source of the problem, and user JSON type terminology instead of Go terminology.

Unfortunately, I don't see a way to do that without writing our own parser.