The ReST API code throws exceptions that turns into HTTP 500 for things that should be HTTP 400 (Bad Request)

An HTTP 500 should be returned by the web server when a major problem has found while processing a request.

If the request handler found an invalid argument, a null argument, etc., it should return a proper HTTP response instead of HTTP 500.

There are several places on the ReST API on which an IllegalArgumentException is thrown. This is not correct and should be fixed.