This commit adds tests for the fixture for when a user is not
authorized (perhaps because the query requires the use of admin
privileges) for a particular query.
This commit adds tests for the sample fixture for when a required
request argument is missing. Also, it moves the sample fixture
to common-error-payloads.md, since this is an error payload that
is common to most requests (except the ones that don't take any
arguments).
We now have a separate page for common error payloads, for example,
the payload for when the client's API key is invalid. All error
payloads that are presented on this page will be tested similarly
to our other non-error sample fixtures.
Eeshan Garg