daemon doesn't return status for certain operations; client times out instead

[tomster]

Symptom

When adding a torrent to transmission-daemon via RPC, removing it again and then trying to re-add it, transmission-daemon doesn't send back an error message but simply prints a debug message transmission-daemon: XXX loaded duplicate torrent. The client, waiting for an answer (either the numerical id of the newly added torrent or a error message as per spec) eventually times out. After restarting the daemon, the torrent can be re-added again (but only once).

Expected behaviour

First of all, re-adding a removed torrent should be possible, however that seems to be covered in #278... This ticket is about not receiving an error message. What I expect is to receive a status message such as "succeeded", "failed", "not-supported", "bad-format", "info" as per the specification​

Note: this symptom occurs also for other set operations such as downlimit, uplimit etc. The client doesn't receive an answer and must manuall verify success by explicitly calling get-downlimit etc.

Steps to reproduce

1. start transmission-daemon
2. add a torrent to transmission-daemon via RPC (the daemon returns the numerical id 1 as per specification.)
3. remove the torrent via RPC
4. repeat step 2 without restarting the daemon first.