[Servicenow] Syncronization job never finishes if getting the attachment list fails

[praveen-elastic]

Bug Description

This issue is similar to that in confluence connector: #2353,

if _attachment_metadata_producer method fails the corresponding END_SIGNAL is never put into the queue. As a consequence, the fetcher_count will never reach 0 and the sync job will remain in the status of "Sync in progress" forever, even after synchronizing all contents.