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.

(cherry picked from commit 7827da05cf5412c0880aafe7c92d7bbb4c173afe)