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

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

@@ -143,6 +143,7 @@ class ErrorPageFilter extends AbstractConfigurableEmbeddedServletContainer imple
 			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());