Adds possibility to raise error when response parsing fails
Google, Okf lookup: fix exception in case response is no valid json

Google and Okf perform additional checks to determine if a request is
valid. It needs to be checked first if json parsing succeeded before
accessing values of it, or else an error will occur.
Use MockHttpResponse instead of String.
Merge pull request #768 from spiderpug/raise-response-parse-error

Adds ability to raise error when response parsing fails