Sla (#11)

Co-authored-by: gorbenko.pavel <gorbenko.pavel@tansu.kz>
Reviewed-on: http://git.tnsu.kz/gorbenko.pavel/tnsu-approval-route/pulls/11

backend/Tnsu.ApprovalRoute.Application/Common/Errors/Errors.cs

@@ -0,0 +1,10 @@
+using FluentResults;
+
+namespace Tnsu.ApprovalRoute.Application.Common.Errors;
+
+public sealed class NotFoundError(string entity, object id)
+    : Error($"{entity} с id {id} не найден.");
+
+public sealed class ValidationError(string message) : Error(message);
+
+public sealed class ConflictError(string message) : Error(message);