Retry "409 Conflict: File with name '...' already exists" errors
If the file is already in the database when the FTS tries to declare it then it will fail with a 409 Conflict error. This shouldn't happen normally as previously the FTS looks up the metadata, which tells it that the file doesn't exist. But there are plainly cases where it sneaks through.
Normally we treat all 4xx errors as fatal. However, for this one we can try looking up the metadata again. This time it should retrieve the metadata from the DB, and we can carry on as normal.
[Copied from RITM0111320]