file_storage: deleting an already-deleted file from a stale tab toasts "0 files deleted" #470

Found during QA of #463. This is pre-existing.

Problem

1. Open /file-storage/ in two tabs.
2. Delete a file in tab A.
3. In tab B, which still lists that file, select it and delete it.

Tab B shows a success toast that reads "0 files deleted", and the stale row disappears. Nothing crashes.

Expected

Tell the user the file was already gone, for example "This file was already deleted", or use a neutral message. A success toast with a count of 0 reads as a failure dressed up as success.