API: when an invalid format is specified, keep the error message shorter (#12401)

so we do not show the stack trace in that case, follows up https://github.com/piwik/piwik/pull/12357

eg on https://demo.piwik.org/?module=API&method=VisitsSummary.getVisits&idSite=1&period=day&date=last10&format=xmls&token_auth=x

core/ExceptionHandler.php

@@ -67,7 +67,13 @@ class ExceptionHandler
     {
         Common::sendHeader('Content-Type: text/html; charset=utf-8');
 
-        echo self::getErrorResponse($exception);
+        try {
+            echo self::getErrorResponse($exception);
+        } catch(Exception $e) {
+            // When there are failures while generating the HTML error response itself,
+            // we simply print out the error message instead.
+            echo $exception->getMessage();
+        }
 
         exit(1);
     }