Return response code from API.

src/api.ts

@@ -1,4 +1,4 @@
-import { initializeApp } from "firebase/app";
+import { FirebaseError, initializeApp } from "firebase/app";
 import { getFirestore, setDoc, doc } from "firebase/firestore"; 
 import { Visitor } from './model/visitor';
 
@@ -16,7 +16,8 @@ initializeApp(firebaseConfig);
 
 const db = getFirestore();
 
-export const addVisitor = async (visitor: Visitor) => {
+export const addVisitor = async (visitor: Visitor): Promise<string> => {
+  try {
     await setDoc(doc(db, "visitors", visitor.name), {
       invitationId: visitor.invitationId,
       allergies: visitor.allergies,
@@ -25,4 +26,8 @@ export const addVisitor = async (visitor: Visitor) => {
     });
 
     await setDoc(doc(db, "submitted", visitor.invitationId), {})
+  } catch(error: any) {
+    return error.code;
+  }
+  return 'success';
 }