fix: validation error messages hidden by generic fallback in ErrorHandler::userMessage

ErrorHandler::userMessage only handled RuntimeException, but all validation
throws in ThesisCreateController and ThesisEditController use plain Exception.
This caused user-friendly messages like 'Le champ Nom/Prénom/Pseudo est requis'
to fall through to the 'Une erreur inattendue est survenue…' generic message.

Fix: add Exception check (after PDOException, since PDOException extends it)
so all validation exceptions pass their message through.

app/src/ErrorHandler.php

@@ -70,9 +70,9 @@ class ErrorHandler
             return self::pdoMessage($e);
         }
 
-        // ── Validation errors (RuntimeException, InvalidArgumentException) ──
+        // ── Validation errors (Exception, RuntimeException) ──────────────
         // These are thrown with user-friendly French messages — pass through.
-        if ($e instanceof \RuntimeException) {
+        if ($e instanceof \Exception) {
             return $e->getMessage();
         }