MQE-118: Handle XML merge failures gracefully

- Added check for false returns from loadXML

Author: aljcalandra

src/Magento/FunctionalTestingFramework/Config/Dom.php

@@ -364,7 +364,10 @@ protected function initDom($xml, $filename = null)
     {
         $dom = new \DOMDocument();
         try {
-            $dom->loadXML($xml);
+            $domSuccess = $dom->loadXML($xml);
+            if (!$domSuccess) {
+                throw new \Exception();
+            }
         } catch (\Exception $exception) {
             throw new ValidationException("XML Parse Error: $filename\n");
         }