欠落している属性を持つファイルを見つけるには、次のファイルを変更できます 
vendor/magento/framework/Config/Reader/Filesystem.php
次のコードを追加 
foreach($configMerger->getDom()->getElementsByTagName('resource') as $element ){
    if (!$element->hasAttribute('title')) {
        var_dump($configMerger->getDom()->saveXML($element));
    }
}
メソッドへ 
protected function _readFiles($fileList)
のように見えるはずです 
protected function _readFiles($fileList)
{
    /** @var \Magento\Framework\Config\Dom $configMerger */
    $configMerger = null;
    foreach ($fileList as $key => $content) {
        try {
            if (!$configMerger) {
                $configMerger = $this->_createConfigMerger($this->_domDocumentClass, $content);
            } else {
                $configMerger->merge($content);
            }
        } catch (\Magento\Framework\Config\Dom\ValidationException $e) {
            throw new \Magento\Framework\Exception\LocalizedException(
                new \Magento\Framework\Phrase("Invalid XML in file %1:\n%2", [$key, $e->getMessage()])
            );
        }
    }
    if ($this->validationState->isValidationRequired()) {
        $errors = [];
        if ($configMerger && !$configMerger->validate($this->_schemaFile, $errors)) {
            foreach($configMerger->getDom()->getElementsByTagName('resource') as $element ){
                if (!$element->hasAttribute('title')) {
                    var_dump($configMerger->getDom()->saveXML($element));
                }
            }
            $message = "Invalid Document \n";
            throw new \Magento\Framework\Exception\LocalizedException(
                new \Magento\Framework\Phrase($message . implode("\n", $errors))
            );
        }
    }
    $output = [];
    if ($configMerger) {
        $output = $this->_converter->convert($configMerger->getDom());
    }
    return $output;
}
これで、間違ったXML要素のダンプを見ることができます。