feat(backend): Eigen error handler toegevoegd.

Hiervoor was ook refactoring aan de exception-klassen nodig.

backend/src/exceptions/exception-with-http-state.ts

@@ -0,0 +1,9 @@
+/**
+ * Exceptions which are associated with a HTTP error code.
+ */
+export abstract class ExceptionWithHttpState extends Error {
+    constructor(public status: number, public error: string) {
+        super(error);
+    }
+}
+