Follow-up from "WIP: Add relationship between blobs and manifest_configurations"

The following discussion from !245 (merged) should be addressed:

• @jdrpereira started a discussion:

  Note: The blob must exist, if not we should halt as we do for layer blobs in L876.

  This shows that we have a hole in the test coverage and that should be addressed (preferably in a followup MR as this one is already too large).