Overview:
 The script for update authorities "Marc Authorities update instructions" export ACTUAL records from SRS to generate .mrc file.
But some of the records have invalid external ids which are linked to not existed inventory records. 
And updating such records through data import causes the job to hang.

We should handle this situation and show discarded message to not break import update process 

Steps to reproduce:

1. Create SRS record with not existed externalId (inventory record id)

2. Try to update MARC record via data-import (matching by 999$s)

Expected Result:
 Update data import job has discarded invalid records with the message "Inventory record not found" and proceed execution.

Actual Result:
 Data import job stuck without any errors and logs inside.

BE: Check for MARC Bib and MARC Holdings updates