Improve ambigous error message for abstract events.

This clarifies the IllegalPluginAccessException that happens when you try to listen to an abstract event with the name of the child class to help the developer to find where he needs to fix his code.
Tested with simplified test case, shows output correctly:
`org.bukkit.plugin.IllegalPluginAccessException: Unable to find handler list for event org.bukkit.event.player.PlayerBucketEvent` 
instead of
`org.bukkit.plugin.IllegalPluginAccessException: Unable to find handler list for event org.bukkit.event.player.PlayerEvent`  
Closes [BUKKIT-4632](https://bukkit.atlassian.net/browse/BUKKIT-4632).