Ensure __toString() catches all error types

In PHP 7.1 `catch (\Exception ...)` does not catch fatal errors. Because this function can possibly throw fatal errors such as "`Call to a member function getRequestFieldName() on null`" which is an instance of `\Error`, we should catch `\Throwable` to ensure the exception makes it into the logger and `$result` is assigned the message.

Author: Tyler Mills

app/code/Magento/Ui/TemplateEngine/Xhtml/Result.php

@@ -115,7 +115,7 @@ public function __toString()
             $this->compiler->compile($templateRootElement, $this->component, $this->component);
             $this->appendLayoutConfiguration();
             $result = $this->compiler->postprocessing($this->template->__toString());
-        } catch (\Exception $e) {
+        } catch (\Throwable $e) {
             $this->logger->critical($e->getMessage());
             $result = $e->getMessage();
         }