Sharing api do not throws on existing share creation

[skjnldsv]

After #25533

Now we can add the same share an unlimited amount of times. Frontend is broken!
Having the api NOT throw on conflict seems like a bad idea to me. Please fix this for 22!
@icewind1991 @nickvergessen @rullzer

[nickvergessen]

Maybe it should just send the notification and stuff and then throw?

[skjnldsv]

This is a blocker for 22 btw :)

[blizzz]

This is a blocker for 22 btw :)

22 24