Add a sanity check for loading Bukkit plugins

[WWOL]

Use a specific message when throwing InvalidPluginException if user tried to load a Bukkit plugin. Could show a message dialogue if not headless?

[darkdiplomat]

Implemented differently https://github.com/CanaryModTeam/CanaryLib/commit/f38e72c8e52f47c8d412ce825b01604d4a0d14fc