ErrorPage for int code does not always work

ErrorPage added to Undertow for the 500 code is not used for the 500 code initiated with the exception.

Example can be found here: https://github.com/isopov/embedded-servlet-container-error-pages/blob/master/src/main/java/com/sopovs/moradanen/errors/UndertowErrorPageCode.java

For both Jetty and Tomcat this case does work: https://github.com/isopov/embedded-servlet-container-error-pages/tree/master/src/main/java/com/sopovs/moradanen/errors