MM-57867 Don't delete existing channel on invitation error (#27143)

* Do not delete existing channels
* don't allow resharung

server/public/model/shared_channel.go

@@ -14,6 +14,7 @@ import (
 var (
 	ErrChannelAlreadyShared = errors.New("channel is already shared")
 	ErrChannelHomedOnRemote = errors.New("channel is homed on a remote cluster")
+	ErrChannelAlreadyExists = errors.New("channel already exists")
 )
 
 // SharedChannel represents a channel that can be synchronized with a remote cluster.