Add not implemented error code

src/boxerror.js

@@ -53,6 +53,7 @@ BoxError.LOGROTATE_ERROR = 'Logrotate Error';
 BoxError.NETWORK_ERROR = 'Network Error';
 BoxError.NGINX_ERROR = 'Nginx Error';
 BoxError.NOT_FOUND = 'Not found';
+BoxError.NOT_IMPLEMENTED = 'Not implemented';
 BoxError.NOT_SIGNED = 'Not Signed';
 BoxError.OPENSSL_ERROR = 'OpenSSL Error';
 BoxError.PLAN_LIMIT = 'Plan Limit';