Fail on inexistent custom trigger

When posting a message starting with a non existent custom trigger, the message gets broadcast to the default destinations. In my opinion this is rather bad because the resulting behavior is not what the user expects, especially when the default action differs substantially from the chosen custom trigger.

A safer behavior would be that when a message starts with the hash ("#") character that signals a custom trigger, if the trigger is non-existent the posting fails with a friendly explicative error message.

As a result of this modification to the custom trigger behavior, someone who actually wants to begin a message with a # that is not the beginning of a custom trigger will have to "escape" the sign with a space for example. But I believe that this is a pretty good trade-off compared to inadvertent broadcasts that could harm the user and force him to tediously clean the target sites.