feat(backend): Eigen error handler toegevoegd.

Hiervoor was ook refactoring aan de exception-klassen nodig.

backend/src/exceptions/unauthorized-exception.ts

@@ -0,0 +1,10 @@
+import {ExceptionWithHttpState} from "./exception-with-http-state.js";
+
+/**
+ * Exception for HTTP 401 Unauthorized
+ */
+export class UnauthorizedException extends ExceptionWithHttpState {
+    constructor(message: string = 'Unauthorized') {
+        super(401, message);
+    }
+}