Prettify JSON output

If the response type is JSON and the whole body would be output anyway,
prettify it using JSON.stringify().
Better checks for undefined data

In a seriously broken test suite I had the problem that there were
strange things: requests that finished successfully, but hat no response
set etc.

This tries to catch such problems as graceful as possible.
Add dpath, suberrors to schema validation errors
Tag as v0.3.0

Adds pretty JSON output and more detail to schema error messages.