Fix: Don't set filter if enable event fails

Furthermore, the session daemon now returns an already exist error if
the event is enabled twice. Since we can now set filter, it's misleading
the user to tell him/her that the event was enabled but the filter
failed and than disabling the already enabled event by a previous
command.

Fixes #363 #364

Signed-off-by: David Goulet <dgoulet@efficios.com>