Allow host app to override login error UI in GetStartedViewController

This reverts some of the changes in 3b46ba8. That commit removed the ability for
the host app to override errors.

I'm applying it a bit differently here though. There was a condition added in
5cadb62 which automatically added an error dialog if the
`WordPressAuthenticator.shared.configuration.enableSignUp` is `false`. The
`sendEmail` is no longer called so I'm adding the override directly in `handleLoginError()`.

Author: shiki

WordPressAuthenticator/Unified Auth/View Related/Get Started/GetStartedViewController.swift

@@ -408,7 +408,22 @@ private extension GetStartedViewController {
                 // username instead.
                 self.showSelfHostedWithError(error)
         } else {
-            self.displayError(error as NSError, sourceTag: self.sourceTag)
+            guard let authenticationDelegate = WordPressAuthenticator.shared.delegate,
+                  authenticationDelegate.shouldHandleError(error) else {
+                self.displayError(error as NSError, sourceTag: self.sourceTag)
+                return
+            }
+
+            /// Hand over control to the host app.
+            authenticationDelegate.handleError(error) { customUI in
+                // Setting the rightBarButtonItems of the custom UI before pushing the view controller
+                // and resetting the navigationController's navigationItem after the push seems to be the
+                // only combination that gets the Help button to show up.
+                customUI.navigationItem.rightBarButtonItems = self.navigationItem.rightBarButtonItems
+                self.navigationController?.navigationItem.rightBarButtonItems = self.navigationItem.rightBarButtonItems
+
+                self.navigationController?.pushViewController(customUI, animated: true)
+            }
         }
     }