Changes the security scanner code to raise exceptions now for non-successful operations. One of the new exceptions raised is MissingParentLayerException, which, when raised, will cause the security worker to perform a full rescan of all parent images for the current layer, before trying once more to scan the current layer. This should allow the system to be "self-healing" in the case where the security scanner engine somehow loses or corrupts a parent layer.
The FakeSecurityScanner mocks out all calls that Quay is expected to make to the security scanner API, and returns faked data that can be adjusted by the calling test case
