Use interface to determine if an exception contains HTML message content.

core/Exceptions/HtmlMessageException.php

@@ -16,15 +16,7 @@ use Exception;
  *
  * @api
  */
-class HtmlMessageException extends Exception
+class HtmlMessageException extends Exception implements HtmlMessageExceptionInterface
 {
-    /**
-     * Returns the exception message.
-     *
-     * @return string
-     */
-    public function getHtmlMessage()
-    {
-        return $this->getMessage();
-    }
+    // empty
 }
\ No newline at end of file

core/Exceptions/HtmlMessageExceptionInterface.php

+<?php
+/**
+ * Piwik - free/libre analytics platform
+ *
+ * @link http://piwik.org
+ * @license http://www.gnu.org/licenses/gpl-3.0.html GPL v3 or later
+ *
+ */
+namespace Piwik\Exceptions;
+
+/**
+ * Exceptions that implement this interface are assumed to have HTML content
+ * in their messages.
+ *
+ * @api
+ */
+interface HtmlMessageExceptionInterface
+{
+    // empty
+}
\ No newline at end of file

core/FrontController.php

@@ -13,6 +13,7 @@ use Exception;
 use Piwik\API\Request;
 use Piwik\API\ResponseBuilder;
 use Piwik\Exceptions\HtmlMessageException;
+use Piwik\Exceptions\HtmlMessageExceptionInterface;
 use Piwik\Http\Router;
 use Piwik\Plugin\Controller;
 use Piwik\Plugin\Report;
@@ -613,8 +614,8 @@ class FrontController extends Singleton
     {
         $debugTrace = $ex->getTraceAsString();
 
-        if (method_exists($ex, 'getHtmlMessage')) {
-            $message = $ex->getHtmlMessage();
+        if ($ex instanceof HtmlMessageExceptionInterface) {
+            $message = $ex->getMessage();
         } else {
             $message = Common::sanitizeInputValue($ex->getMessage());
         }