Log exceptions in ErrorPageFilter

For some reason I thought that the DispatcherServlet default handler
would log all exceptions, so if we did it in the ErrorPageFilter it
would lead to duplicates. That appears not to be the case.

Fixes gh-1427

Author: Dave Syer

spring-boot/src/main/java/org/springframework/boot/context/web/ErrorPageFilter.java

@@ -143,6 +143,7 @@ private void handleException(HttpServletRequest request,
 			rethrow(ex);
 			return;
 		}
+		logger.error("Forwarding to error page", ex);
 		setErrorAttributes(request, 500, ex.getMessage());
 		request.setAttribute(ERROR_EXCEPTION, ex);
 		request.setAttribute(ERROR_EXCEPTION_TYPE, type.getName());