Throw an error if a specified value does exist in the prior response

// Get the Message that precedes this Script.
var msg = $context.currentItem.previousItem;

// Get the response to the Message.
var response = msg.getResponse(msg.RESPONSE_HTTP_BODY);

// Was there a response?
if (response == null)
{
// There was no response.

// Output an informational message.
$context.result.postMessage($context.result.LEVEL_INFO, "Message " + msg.name + " received no response to validate.");
}
else
{
// There was a response.

// Get the value of the Global Property.
// var stringToFind = $globalprop.value("Global property list name here", "Global property name here");

// string to find; hard coded
var stringToFind = "seeing this error because you have";

// Determine if the value exists in the response.
var searchIndex = response.indexOf(stringToFind);

// If the value does exist in the response, output an error.
if (searchIndex > -1)
{
var details = "Response:\n" + response + "\n\nValue searched for:\n" + stringToFind;
$context.result.postMessage($context.result.LEVEL_ERROR, "Error found in response to Message " + msg.name, details);
}
}

msg.clearResponse();