During Spaces migration, if attachments fail to migrate for any reason, notify the failure in CCMA UI

Issue Summary

During migration of spaces with CCMA, attachments may fail to migrate due to several underlying issues, such as OS open file limit on Confluence Server. This error is not indicated in CCMA migration UI. This can lead to customers assuming the migration has succeeded and can be cause of post-migration issues after go-live.

This is critical still as there is no easy way to migrate attachments-only, after migration.

This is reproducible on Data Center: (no)

Steps to Reproduce

1. Set OS open file limit to a very low value (for the purpose of this reproduction)
2. Migration spaces with CCMA
3. After migration, some attachments would have failed to migrate

Expected Results

Either:

1. Attachments are successfully migrated, OR
2. Any failure to migrate attachments is highlighted in CCMA UI

Actual Results

To cite one example seen in a recent ticket, the below exception may be thrown in the atlassian-confluence.log file:

Caused by: java.nio.file.FileSystemException: /servers/tomcat/confluence-data/attachments/ver003/194/40/106790944/68/154/116654318/137010785/1: Too many open files

However, this is not highlighted in CCMA migration wizard

Workaround

Currently being tested:

1. Run attachment-only migration after fixing OS-level open file limit
2. Contact Atlassian Support to run a fix from DB end