Add UI error message when IAP succeeds but receipt verification fails

This should not be visible to end-users on normal circumstances, but we
should regardless show an error message if something goes wrong with the
IAP receipt verification, to prompt them to contact support.

Testing
-------

PASS

Device: iPhone simulator
iOS: 17.2
Damus: This commit
damus-api: d3801376fa204433661be6de8b7974f12b0ad25f
Setup:
- Local Testing server
- Xcode StoreKit environment
Steps:
1. Set MOCK_VERIFY to false on the server (that means receipt verification will fail on Xcode environment)
2. Try to make IAP purchase. Error should appear on UI
3. Set MOCK_VERIFY to true on the server and restart StoreKit environment (Receipt validation will always work)
5. Try to make IAP purchase. Onboarding flow should go as normal with no error messages. PASS

Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damus/Views/Purple/DamusPurpleView.swift

@@ -144,6 +144,12 @@ struct DamusPurpleView: View, DamusPurpleStoreKitManagerDelegate {
                     // If account is no longer active or was purchased via IAP, then show IAP purchase/manage options
                     if let account_uuid {
                         DamusPurpleView.IAPProductStateView(products: products, purchased: purchased, account_uuid: account_uuid, subscribe: subscribe)
+                        if let iap_error {
+                            Text(String(format: NSLocalizedString("There has been an unexpected error with the in-app purchase. Please try again later or contact support@damus.io. Error: %@", comment: "In-app purchase error message for the user"), iap_error))
+                                .foregroundStyle(.red)
+                                .multilineTextAlignment(.center)
+                                .padding(.horizontal)
+                        }
                     }
                     else {
                         ProgressView()