Event handler errors that cannot be covered by `RetryManager`

[ok300]

The various branches of handle_put_event can fail for all sorts of reasons that cannot be addressed later by RetryManager, like:

• missing permissions (to write a file)
• file already exists (when saving attachment file)
  • Event indexing: fix edge-case when reindexing skips attachments #736

We need a clean way to propagate and handle these errors. The main distinction is these cannot be covered by RetryManager.