Admin controller, Locations controller, requests to add available countries, request to get available countries

SchengenVisaApi/ApplicationLayer/GeneralExceptions/AlreadyExistsException.cs

@@ -0,0 +1,4 @@
+namespace ApplicationLayer.GeneralExceptions
+{
+    public class AlreadyExistsException(string message) : ApiException(message);
+}

SchengenVisaApi/ApplicationLayer/GeneralExceptions/ApiException.cs

@@ -0,0 +1,4 @@
+namespace ApplicationLayer.GeneralExceptions
+{
+    public class ApiException(string message) : Exception(message);
+}