Macros: new mk_exception(), similar to mk_bug() but without conditional

Signed-off-by: Eduardo Silva <eduardo@monkey.io>
Request: trigger an exception when an entry already exists on the rbtree

Signed-off-by: Eduardo Silva <eduardo@monkey.io>
Scheduler: trigger exception when registering a client that already exists

Signed-off-by: Eduardo Silva <eduardo@monkey.io>