message: Make UDD appear when UDE on uploading a file
  This has highlighted the fact that an unsent image looks very much like a sent image (https://github.com/vector-im/riot-web/issues/3391). Also, the "Resend" status bar doesn't appear when an image is unsent.
diff: @@ -915,8 +915,6 @@ module.exports = React.createClass({
  },
  uploadFile: function(file) {
  - var self = this;
  -
  if (MatrixClientPeg.get().isGuest()) {
  var NeedToRegisterDialog = sdk.getComponent("dialogs.NeedToRegisterDialog");
  Modal.createDialog(NeedToRegisterDialog, {
  @@ -928,8 +926,16 @@ module.exports = React.createClass({
  ContentMessages.sendContentToRoom(
  file, this.state.room.roomId, MatrixClientPeg.get()
  - ).done(undefined, function(error) {
  - var ErrorDialog = sdk.getComponent("dialogs.ErrorDialog");
  + ).done(undefined, (error) => {
  + if (error.name === "UnknownDeviceError") {
  + dis.dispatch({
  + action: 'unknown_device_error',
  + err: error,
  + room: this.state.room,
  + });
  + return;
  + }
  + const ErrorDialog = sdk.getComponent("dialogs.ErrorDialog");
  Modal.createDialog(ErrorDialog, {
  title: "Failed to upload file",
  description: error.toString()
  