Add API level support for error message and redirect during registration flow

osu.Game/Online/API/APIAccess.cs

@@ -329,7 +329,29 @@ namespace osu.Game.Online.API
             {
                 try
                 {
-                    return JObject.Parse(req.GetResponseString().AsNonNull()).SelectToken("form_error", true).AsNonNull().ToObject<RegistrationRequest.RegistrationRequestErrors>();
+                    return JObject.Parse(req.GetResponseString().AsNonNull()).SelectToken(@"form_error", true).AsNonNull().ToObject<RegistrationRequest.RegistrationRequestErrors>();
+                }
+                catch
+                {
+                    try
+                    {
+                        // attempt to parse a non-form error message
+                        var response = JObject.Parse(req.GetResponseString().AsNonNull());
+
+                        string redirect = (string)response.SelectToken(@"url", true);
+                        string message = (string)response.SelectToken(@"error", false);
+
+                        if (!string.IsNullOrEmpty(redirect))
+                        {
+                            return new RegistrationRequest.RegistrationRequestErrors
+                            {
+                                Redirect = redirect,
+                                Message = message,
+                            };
+                        }
+
+                        // if we couldn't deserialize the error message let's throw the original exception outwards.
+                        e.Rethrow();
                     }
                     catch
                     {
@@ -337,6 +359,7 @@ namespace osu.Game.Online.API
                         e.Rethrow();
                     }
                 }
+            }
 
             return null;
         }

osu.Game/Online/API/RegistrationRequest.cs

@@ -23,6 +23,16 @@ namespace osu.Game.Online.API
 
         public class RegistrationRequestErrors
         {
+            /// <summary>
+            /// An optional error message.
+            /// </summary>
+            public string? Message;
+
+            /// <summary>
+            /// An optional URL which the user should be directed towards to complete registration.
+            /// </summary>
+            public string? Redirect;
+
             public UserErrors? User;
 
             public class UserErrors