Human Task Output Validation chokes upon wrong json

[tpetter]

If the service that is being invoked from within the engine does not respond with a valid JSON object structure - i.e., with opening and closing curly brackets { ... } - then the TaskOutputValidator ends up either a ClassCastException or a "technical failure" without much further info.
The error handling is expected to be more robust.

[tpetter]

if no response is given JSONReader throws JSONParseFailure, but it can be more intelligent when there is no token.