Ruby JSONError is in the JSON module

This smoke test was failing with an initialised constant error rather
than reporting a JSON error.
Add more detail to backdrop smoke test failure

Add the error message and read response to JSON parse failures in the
backdrop smoke test.

In Kibana all the requests are claiming to be fine so this is very
strange. I'd like a bit more detail on it.
Merge pull request #332 from alphagov/fix-backdrop-smoke-test

Fix backdrop smoke test