refactor #209 Fix wrong type error if the error in some cases (Nek-)

This PR was merged into the 1.0-dev branch.

Discussion
----------

Problem
-------

The raised error may be a throwable since the user is supposed to add its custom code with handlers. But since this is type hinted Exception, the real failure will be hidden by this new TypeError.

Solution
--------

Accept Throwable instead.

Fixes #208

Commits
-------

f998e0e Fix wrong type error if the error in some cases

src/Action/Admin/RefundUnitsAction.php

@@ -70,7 +70,7 @@ public function __invoke(Request $request): Response
         ));
     }
 
-    private function provideErrorMessage(\Exception $previousException): void
+    private function provideErrorMessage(\Throwable $previousException): void
     {
         if ($previousException instanceof InvalidRefundAmountException) {
             $this->session->getFlashBag()->add('error', $previousException->getMessage());