Better handling of user-space exceptions in events handlers and formatters

This is a resubmit of #824. It was closed without comment, though I see I didn't provide a very good explanation of the issue. See below for that:

Exception thrown from these handlers and formatters cause SlickGrid to go into a "broken" state where it responds very erratically to user input. I can try to create a test case if you want. Wrapping this user-code in exception handlers prevents this, and makes SG more robust.