Basket should return an error when passing unknown data

[alexgibson]

Ran into this error recently whilst working on mozilla/bedrock#11944

When posting incorrect labelled data to /news/subscribe/ (in this case it was fmt=H instead of format=H), basket gives a success message but then fails silently and does not generate a confirmation email. Instead, it should return an error so it's clear to clients that something went wrong.

Expected result:

{"status": "error", "desc": "Unknown data"} (or something along these lines)

Actual result:

{"status": "ok"}