Handle badly formatted cookie value

[mikemonteith]

We should be handling malformed cookie values. This is unlikely to happen in the wild, but there is a possibility that some bad host code or browser bug could set the value of our nhsuk-cookie-consent cookie to something that can't be JSON parsed.

JSON.parse will throw an exception if the input is badly formed.

We should catch any JSON.parse exceptions and reset the cookie value to something sensible.