Show alert on error, prevent sending request if one is already in flight.

Mobile Buy SDK Sample Apps/Sample App Customers/Sample App Customers/AccountViewController.swift

@@ -85,6 +85,6 @@ extension AccountViewController: AuthenticationDelegate {
     }
     
     func authenticationDidFailWithError(error: NSError?) {
-        print("Failed to authenticate customer: \(error)")
+        UIAlertView(title: "Error", message: error?.localizedDescription, delegate: nil, cancelButtonTitle: "OK").show()
     }
 }
\ No newline at end of file

Mobile Buy SDK Sample Apps/Sample App Customers/Sample App Customers/LoginViewController.swift

@@ -51,6 +51,8 @@ class LoginViewController: UITableViewController {
     //  MARK: - Actions -
     //
     private func loginUser() {
+        guard !self.actionCell.loading else { return }
+        
         let credentials  = BUYAccountCredentials(items: [
             BUYAccountCredentialItem(email: self.email),
             BUYAccountCredentialItem(password: self.password),

Mobile Buy SDK Sample Apps/Sample App Customers/Sample App Customers/SignupViewController.swift

@@ -57,6 +57,8 @@ class SignupViewController: UITableViewController {
     //  MARK: - Actions -
     //
     private func createUser() {
+        guard !self.actionCell.loading else { return }
+        
         let credentials = BUYAccountCredentials(items: [
             BUYAccountCredentialItem(firstName: self.firstName),
             BUYAccountCredentialItem(lastName: self.lastName),