Image errors block the tool
When creating the asketch files, i encountered a bug when the headless browser could not reach the images referenced in the html file. I debugged a bit, and made a few changes to fixImageFill.js. I am not very familiar with Sketch Plugins, so thought I would share my hack here with you, so someone more competent could implement the right solution. 

I changed the function fixImageFill to the following:

```js
export function fixImageFill(layer, fill) {
  if (!fill.image || !fill.image.url) {
    return null;
  }

  let {type: imageType, data: imageData} = getImageDataFromUrl(fill.image.url);
  let error = false;
  if (imageType === IMG_ERROR || imageType === IMG_UNSUPPORTED) {
    imageType = IMG_BLOB;
    error = true;
    imageData = getErrorImage();
  }

  if (imageType === IMG_BLOB) {

    let nsImage;
    if(!error){
      nsImage = NSImage.alloc().initWithData(imageData);
    } else {
      nsImage = imageData;
    }
    let img = null;

    if (MSImageData.alloc().initWithImage_convertColorSpace !== undefined) {
      img = MSImageData.alloc().initWithImage_convertColorSpace(nsImage, false);
    } else {
      img = MSImageData.alloc().initWithImage(nsImage);
    }

    const data = img.data().base64EncodedStringWithOptions(NSDataBase64EncodingEndLineWithCarriageReturn);
    const sha1 = img.sha1().base64EncodedStringWithOptions(NSDataBase64EncodingEndLineWithCarriageReturn);

    fill.image.data = {_data: data};
    fill.image.sha1 = {_data: sha1};

    delete fill.image.url;
  } else if (imageType === IMG_SVG) {
    const svgLayer = {
      x: layer.frame.x,
      y: layer.frame.y,
      width: layer.frame.width,
      height: layer.frame.height,
      rawSVGString: imageData
    };

    fixSVGLayer(svgLayer);

    // we are replacing the parent layer with SVGLayer
    replaceProperties(layer, svgLayer);
  }

  return imageType;
}
```