[Glitch] Add missing rejection handling for Promises

Port missing parts from 2c51bc0ca5 to glitch-soc

app/javascript/flavours/glitch/features/ui/components/embed_modal.js

@@ -10,6 +10,7 @@ export default class EmbedModal extends ImmutablePureComponent {
   static propTypes = {
     url: PropTypes.string.isRequired,
     onClose: PropTypes.func.isRequired,
+    onError: PropTypes.func.isRequired,
     intl: PropTypes.object.isRequired,
   }
 
@@ -35,6 +36,8 @@ export default class EmbedModal extends ImmutablePureComponent {
       iframeDocument.body.style.margin = 0;
       this.iframe.width  = iframeDocument.body.scrollWidth;
       this.iframe.height = iframeDocument.body.scrollHeight;
+    }).catch(error => {
+      this.props.onError(error);
     });
   }