Adding guards and more tests (#68)

* Adding single entity by spec method to repository

* Adding guards and more unit tests

src/ApplicationCore/Exceptions/BasketNotFoundException.cs

@@ -0,0 +1,23 @@
+using System;
+
+namespace ApplicationCore.Exceptions
+{
+    public class BasketNotFoundException : Exception
+    {
+        public BasketNotFoundException(int basketId) : base($"No basket found with id {basketId}")
+        {
+        }
+
+        protected BasketNotFoundException(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) : base(info, context)
+        {
+        }
+
+        public BasketNotFoundException(string message) : base(message)
+        {
+        }
+
+        public BasketNotFoundException(string message, Exception innerException) : base(message, innerException)
+        {
+        }
+    }
+}