feat(backend): Added endpoint to fetch HTML version of learning object (from Dwengo backend)

Also refactored a bit to make this easier.

backend/src/exceptions.ts

@@ -0,0 +1,21 @@
+/**
+ * Exception for HTTP 400 Bad Request
+ */
+export class BadRequestException extends Error {
+    public status = 400;
+
+    constructor(error: string) {
+        super(error);
+    }
+}
+
+/**
+ * Exception for HTTP 404 Not Found
+ */
+export class NotFoundException extends Error {
+    public status = 404;
+
+    constructor(error: string) {
+        super(error);
+    }
+}