Fixed BUKKIT-2613

As described in [BUKKIT-2613](https://bukkit.atlassian.net/browse/BUKKIT-2613), uncaught exceptions in the conversations API lead to `End of stream` disconnects without any information. This pull request wraps `Prompt`-calls in try-blocks. When an exception is thrown, it will be logged and the conversation will be abandoned (instead of disconnecting the player).