public GraphqlServiceBuilder errorHandler(GraphqlErrorHandler errorHandler) {
        requireNonNull(errorHandler, "errorHandler");
        if (this.errorHandler == null) {
            this.errorHandler = errorHandler;
        } else {
            this.errorHandler = this.errorHandler.orElse(errorHandler);
        }
        return this;
    }
Adds the {@link GraphqlErrorHandler}. If multiple handlers are added, the latter is composed with the
former one using {@link GraphqlErrorHandler#orElse(GraphqlErrorHandler)}.

<p>If not specified, {@link GraphqlErrorHandler#of()} is used by default.
graphql/src/main/java/com/linecorp/armeria/server/graphql/GraphqlServiceBuilder.java