Throw an error when attempting to mock data that doesn't exist
Throw an error when attempting to mock data that doesn't exist

This will allow users to have their specs fail fast and fail hard
when they provide an invalid graphql type override. Previous the
invalid type would be silently ignored, leading to subtle bugs in
unit tests.

Test Plan:
  - Jenkins passes

flag = none

Change-Id: I4013621b6347c86ca7dcdaa2056910d0a3f67c22
Reviewed-on: https://gerrit.instructure.com/212326
Tested-by: Jenkins
Reviewed-by: Ryan Norton <65299b45178f94900ff304765e8fbf07bd13b331@instructure.com>
QA-Review: Landon Gilbert-Bland <60d9ff23682c863d36b48b4c32c5ce5f434d29c0@instructure.com>
Product-Review: Landon Gilbert-Bland <60d9ff23682c863d36b48b4c32c5ce5f434d29c0@instructure.com>

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