Optionally handle exceptions in YStore

[Zsailer]

Adding an issue to track some follow-up work mentioned here: #33 (comment)

The intent here is to handle exception where they happen, rather than outer layers. In this case, the exception could handle in a Ystore, but it wouldn't be handled until the YRoom catches the exception. We should follow the exception_handler pattern introduced in #31 to handle exceptions in the YRoom.