Fix wrong type error if the error in some cases

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

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());