improve output on invalid JSON in an example

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](https://github.com/zaach/jsonlint)

This seems in the same vein as issue #19.

Start by reproducing the invalid-JSON example described in the issue and compare the behavior with issue #19. Done means the error clearly identifies the example rather than the schema and provides useful information about the invalid JSON, potentially using json-lint as suggested.

Written by the indexing model from the issue text.