Improve cascade file loading and error handling for js tutorials

doc/js_tutorials/js_assets/utils.js

@@ -19,6 +19,24 @@ function Utils(errorOutputId) { // eslint-disable-line no-unused-vars
         node.parentNode.insertBefore(script, node);
     };
 
+    this.createFileFromUrl = function(path, url, callback) {
+        let request = new XMLHttpRequest();
+        request.open('GET', url, true);
+        request.responseType = 'arraybuffer';
+        request.onload = function(ev) {
+            if (request.readyState === 4) {
+                if (request.status === 200) {
+                    let data = new Uint8Array(request.response);
+                    cv.FS_createDataFile('/', path, data, true, false, false);
+                    callback();
+                } else {
+                    self.printError('Failed to load ' + url + ' status: ' + request.status);
+                }
+            }
+        };
+        request.send();
+    };
+
     this.loadImageToCanvas = function(url, cavansId) {
         let canvas = document.getElementById(cavansId);
         let ctx = canvas.getContext('2d');