Call event when boat is destroyed by falling. Fixes BUKKIT-5500
Check that item in hand is not null after book event. Fixes BUKKIT-5443
Catch exceptions when abandoning conversions. Fixes BUKKIT-5436

If a conversation is abandoned due to a player disconnecting and an
exception is thrown in a ConversationAbandonedListener, the server will
crash. This commit prevents the exception from propagating further up
the stack and instead just logs the error.