Handle additional bodyParser errors

[matthew-white]

If the size of a JSON request body exceeds the bodyParser limit, then bodyParser will throw an error with a status property of 413. However, Backend will then return a 500 error, not a 413. The error has the message, "Completely unhandled exception: request entity too large". I think it would be ideal for Backend to return a 413 Problem in this case. (See #446 for some more details about this error.)

We already handle bodyParser errors of type entity.parse.failed:

central-backend/lib/http/service.js

Lines 88 to 91 in 5f86e22

	} else if ((error != null) && (error.type === 'entity.parse.failed')) {
	// catch body-parser middleware problems. we only ask it to parse JSON, which
	// isn't part of OpenRosa, so we can assume a plain JSON response.
	defaultErrorWriter(Problem.user.unparseable({ format: 'json', rawLength: error.body.length }), request, response);

Given that, I think we could do something similar with errors of type entity.too.large. It would probably be useful to also take a look at the list of bodyParser errors to see if there are others to account for. (Is there a generic way to handle any bodyParser error?)