How can I prevent Google Apps Scripts from returning HTML to slack after processing a custom slash command?
<p>I am using a custom slash command in Slack to add data to a Google Doc via Google Apps Scripts, however, when the script finishes running (successfully), Google Script sends this to Slack, which users will interpret as an error:</p>

<pre><code>&lt;DOCTYPE html&gt;&lt;html&gt;&lt;head&gt;&lt;link rel="shortcut icon" href="//ssl.gstatic.com/docs/script/images/favicon.ico"&gt;&lt;title&gt;Error&lt;/title&gt;&lt;style type="text/css"&gt;body {background-color: #fff; margin: 0; padding: 0;}.errorMessage {font-family: Arial,sans-serif; font-size: 12pt; font-weight: bold; line-height: 150%; padding-top: 25px;}&lt;/style&gt;&lt;/head&gt;&lt;body&gt;&lt;div&gt;&lt;img src="//ssl.gstatic.com/docs/script/images/logo.png"&gt;&lt;/div&gt;&lt;center&gt;The script completed but did not return anything.&lt;/center&gt;&lt;/body&gt;&lt;/html&gt;
</code></pre>

<p>How can I prevent this message from being sent to Slack?</p>

<pre><code>function doPost(request) {
  var sheets = SpreadsheetApp.openById('id-would-go-here');
  var params = request.parameters;

  var nR = getNextRow(sheets) + 1;

  if (params.token == "token-would-go-here") {

    // FALL BACK TO DEFAULT TEXT IF NO ORDER PROVIDED
    var order   = params.text || "No Order Entered";
    var employee = params.user_name || "Name Error";

    // RECORD TIMESTAMP AND USER NAME IN SPREADSHEET
    sheets.getRangeByName('date').getCell(nR,1).setValue(new Date());
    sheets.getRangeByName('employee').getCell(nR,1).setValue(employee);

    // RECORD UPDATE INFORMATION INTO SPREADSHEET
    sheets.getRangeByName('order').getCell(nR,1).setValue(order);

    var channel = "lunch-orders";

    postResponse(channel,order,employee);

    var eph_url = params.response_url; 
    var eph_response = UrlFetchApp.fetch(eph_url,200);

  } else {
    return;
  }
}

function postResponse(channel, order, employee) {

  var payload = {
    "channel": "#lunch-orders",
    "username": "Lunch Order",
    "link_names": 1,
    "attachments":[
       {
          "fallback": "A lunch order was placed, but the display here is a little messed up. Check the Google Doc to view the order.",
          "mrkdwn_in": ["pretext"],
          "color": "good",
          "fields":[
             {
                "title":"" + employee + "'s Lunch Order",
                "value": "" + order + "",
                "short":false
             }
          ]
       }
    ]
  };

  var url = 'incoming-webhook-url-goes-here';
  var options = {
    'method': 'post',
    'payload': JSON.stringify(payload)
  };

  var response = UrlFetchApp.fetch(url,options);
}
</code></pre>