Add HttpError.toJSON() method

[samchon]

New method HttpError.toJSON() method for parsing response body.

export class HttpError extends Error {
    public readonly method: "GET" | "DELETE" | "POST" | "PUT" | "PATCH";
    public readonly path: string;
    public readonly status: number;

    pubilc toJSON<T>(): HttpError.IProps<T>;
}
export namespace HttpError {
    export interface IProps<T> {
        method: "GET" | "DELETE" | "POST" | "PUT" | "PATCH";
        path: string;
        status: number;
        message: T;
    }
}