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.
Following this change, anytime a layer is indexed by the security scanner, we only send notifications out if the layer previously had a security_indexed_engine value of `-1`, thus ensuring it has *never* been indexed previously. This will allow us to change to version of the security scanner upwards, and have all the images be re-indexed, without firing off notifications in a spammy manner.
