Add NotFoundException for 404 status

Octokit/Exceptions/NotFoundException.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Net;
+using System.Runtime.Serialization;
+
+namespace Octokit
+{
+#if !NETFX_CORE
+    [Serializable]
+#endif
+    [SuppressMessage("Microsoft.Design", "CA1032:ImplementStandardExceptionConstructors",
+        Justification = "These exceptions are specific to the GitHub API and not general purpose exceptions")]
+    public class NotFoundException : ApiException
+    {
+        public NotFoundException(IResponse response) : this(response, null)
+        {
+        }
+
+        public NotFoundException(IResponse response, Exception innerException) : base(response, innerException)
+        {
+            Debug.Assert(response != null && response.StatusCode == HttpStatusCode.NotFound,
+                "NotFoundException created with wrong status code");
+        }
+
+        #if !NETFX_CORE
+        protected NotFoundException(SerializationInfo info, StreamingContext context)
+            : base(info, context)
+        {
+        }
+#endif
+    }
+}