Attachments with incorrect base64 contentUrl in sendActivity fails silently

Versions

What package version of the SDK are you using: 4.0.8 (botbuilder, botbuilder-dialogs)
What nodejs version are you using: v.8.12.0
What browser version are you using: Chrome v69
What os are you using: Windows 10

Describe the bug

When trying to create an attachment that is intentionally corrupt (via an incorrect base64 url), the bot fails silently without throwing an error.

To Reproduce

1. Create bot with command to trigger send activity with an attachment with the following code:

  try {
    console.log('Start');

    await context.sendActivity({
      type: 'message',
      attachments: [{
        contentType: 'text/plain',
        contentUrl: 'data:image/png;base64,abc\nabc',
        name: 'dot.png'
      }]
    });

    console.log('Succeeded');
  } catch (err) {
    console.log(err);
    throw err;
  }

And on the BotFrameworkAdapter instance, register an onTurnError handler like so:

adapter.onTurnError = async (context, error) => {
  console.log(error);
  await context.sendActivity(`Sorry, ran into an error!\n  ${ error.message }`);
}

2. Run bot, send command utterance to send activity
3. Sending the command failed
   Note that there is no error in the console, and the line console.log("Succeeded"); is never reached. Similarly, onTurnError is never triggered.

The message fails silently, and after timing out, the Web Chat instance logs a 502 Bad Gateway error.

Sending other messages pass through successfully.

Expected behavior

Since the contentUrl is invalid, the bot should try and catch an error when this command is initiated.

[bug]