Specify code of 500.1 for "Completely unhandled exception"?

If an error is thrown that is not a Problem, then in some cases, Backend will return a 500 error. The response body will be a JSON object with `message` and `details` properties. `message` will start with "Completely unhandled exception", and `details` will hold the stack trace if there is one. getodk/central#788 is an example of an error like this: the full message there is "Completely unhandled exception: request entity too large". The logic for errors of this kind seems to be located in `defaultErrorWriter()`:

https://github.com/getodk/central-backend/blob/5f86e228d88f4b5d38ab876815581e1d4134e1f7/lib/http/endpoint.js#L190-L213

I'm wondering whether we should add a `code` property to the JSON object so that the response is a full-fledged Problem. I think a code of 500.1 would make sense. Frontend knows how to handle errors that are Problems: by default, it will show the Problem message to the user. However, when an error response isn't a Problem, Frontend shows a generic error message, for example, "Something went wrong: error code 500." My instinct is that if we have a more informative error message, it would be useful to show that to the user.