Show unhandled exceptions prominently

Currently, to avoid flooding logs in case of syntax errors etc, unhandled exceptions are not logged. However, this is a usability issue because when developing handlers, if an exception is thrown and not caught, it terminates the handler execution and there is no obvious mark this happened, except for incrementing of a stat.

We need do better here, and I recommend we log unhandled errors but rate limit by handler_name:line_number to avoid flooding logs. We also need to fetch a list of 'n' most recently thrown exceptions and indicate then in the JS editor UI with breadcrumbs (like we do for syntax errors), and hovering on it should show exception details.