GuardAppClient: Tests fixed and validation message improved.

src/Squidex.Domain.Apps.Entities/Apps/Guards/GuardAppClients.cs

@@ -25,7 +25,7 @@ namespace Squidex.Domain.Apps.Entities.Apps.Guards
                 }
                 else if (clients.ContainsKey(command.Id))
                 {
-                    error(new ValidationError("Client id already exists."));
+                    error(new ValidationError($"A client with id '{command.Id}' has already been added."));
                 }
             });
         }

tests/Squidex.Domain.Apps.Entities.Tests/Apps/Guards/GuardAppClientsTests.cs

@@ -36,7 +36,7 @@ namespace Squidex.Domain.Apps.Entities.Apps.Guards
             var clients_1 = clients_0.Add("android", "secret");
 
             ValidationAssert.Throws(() => GuardAppClients.CanAttach(clients_1, command),
-                new ValidationError("Client id already exists.", "Id"));
+                new ValidationError("A client with id 'android' has already been added."));
         }
 
         [Fact]