DB could contain source 'MARC' instances without related MARC records, so we should handle this. This can happen because of users manually deleting SRS MARCs, but not maintaining the Instance Source field, or from errored import jobs.

Now, when the user tries to update such an instance, error returns. The instance is not editable. Only way to overlay it or edit it is to manually change the Source to FOLIO via db change

If possible to resolve in a subsequent import (e.g. an Inventory Single Record Import telling FOLIO to link this particular Instance to this particular incoming MARC Bib), resolve the source/SRS link problem.
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Possible workaround: Correct the source of the Instance to FOLIO (via the API?) and then run the import

BE NOTES
Ignore (log as a warning) 404 response code from SRS at ReplaceInstanceEventHandler (241 line)