title: Glide - call method after fallback or error when trying load photo
body: <p>Glide - call method after fallback or error when trying load photo.</p>
  
  <p>Hi!</p>
  
  <p>Is there any a way to check if <code>Glide</code> load photo from link or use <code>fallback</code>/<code>error</code> when link isn't valid or photo isn't available?</p>
  
  <p>I mean, I want to call a method (load other photo) when Glide doesn't load photo.</p>
  
  <p>This is my Glide e.g.:</p>
  
  <pre><code>Glide
          .with(mActivity)
          .load(news.getPagemap().getCseThumbnail().get(0).getSrc())
          .fallback(R.drawable.bg_gradient)
          .error(R.drawable.bg_gradient)
          .centerCrop()
          .crossFade()
          .diskCacheStrategy(DiskCacheStrategy.SOURCE)
          .into(holder.photo);
  </code></pre>
  
  <p>I tried to compare <code>ConstantValues</code> - <code>holder.photo.getDrawable().getConstantState().equals(mActivity.getResources().getDrawable(R.drawable.bg_gradient).getConstantState())</code> but got <code>NullPointerException</code>.</p>