improve output on invalid JSON in an example

[bpytlik]

When an example isn't valid JSON, the error you get looks like this:
AssertionError: Invalid JSON:
undefined
Error: expected [Function] to not throw an error but 'SyntaxError: Unexpected string' was thrown

Followed by a stack trace. It would be nice if:
a) the error identified that it was the example and not the json schema that was invalid.
b) the error provided more information about what was invalid, perhaps via something like json-lint

This seems in the same vein as issue #19.

[cybertk]

See PR #29