[NT-649] Errored pledges query

[Scollaco]

📲 What

• Adds a query to fetch errored pledges from graphQL.

🤔 Why

• We need this query to show a list of errored pledges on the ActivitiesViewController and allow users to fix them.

🛠 How

• Created a new model GraphBacking
• Added the required fields on the GraphSchema
• Added output on ActivityViewModel that emits the list of errored pledges
• Added tests

JIRA ticket

✅ Acceptance criteria

Testing with Charles

• Open the app and tap the Activity tab. A list of errored backings (if any), should be visible on the response.

If there's no errored pledges

• Add a breakpoint to intercept the response once the ActivitiesViewController is open and change the status of backings to errored. The erroredBackings output on ActivitiesViewModel should emit a list with the errored pledges.

[justinswart]

Cool lgtm, had minor comments!

[justinswart]

If we use the non-failable Data initializer then we don't need to force-unwrap later on:

let data = Data(jsonString.utf8)

[justinswart]

I see the back-end also has a status authentication_required 🤔