Update the BasicErrorController so that it no longer needs to implement
@ControllerAdvice or have an @ExceptionHandler method.

A new ErrorAttributes interface is now used to obtain error details,
the DefaultErrorAttributes implementation uses a
HandlerExceptionResolver to obtain root exception details if the
`javax.servlet.error.*` attributes are missing.

This change also removes the need for the extract(...) method on
ErrorController as classes such as WebRequestTraceFilter can
now use the ErrorAttributes interface directly.

See gh-839, gh-538
Fixes gh-843