implementet ErrorMessages

Apollon.WPF/ViewModels/OverviewViewModel.cs

@@ -29,6 +29,23 @@ namespace Apollon.WPF.ViewModels
             }
         }
 
+        private string _errorMessage;
+        public string ErrorMessage
+        {
+            get
+            {
+                return _errorMessage;
+            }
+            set
+            {
+                _errorMessage = value;
+                OnPropertyChanged(nameof(ErrorMessage));
+                OnPropertyChanged(nameof(HasErrorMessage));
+            }
+        }
+
+        public bool HasErrorMessage => !string.IsNullOrEmpty(ErrorMessage);
+
         public ICommand AddTournamentCommand { get; }
         public ICommand LoadTournamentsCommand { get; }
         public ICommand NavigateNavBarCommand { get; }