Update errors

Author: Andrew Omondi

docs/errors.md

@@ -12,8 +12,8 @@ try
 }
 catch (ODataError odataError)
 {
-    Console.WriteLine(odataError.Error.Code);
-    Console.WriteLine(odataError.Error.Message);
+    Console.WriteLine(odataError.Error?.Code);
+    Console.WriteLine(odataError.Error?.Message);
     throw;
 }
 ```
@@ -24,7 +24,7 @@ catch (ODataError odataError)
 You can check the status code that caused the error as below.
 
 ```csharp
-catch (ODataError odataError) when (odataError.ResponseStatusCode.Equals(404))
+catch (ODataError odataError) when (odataError.ResponseStatusCode.Equals(HttpStatusCode.NotFound))
 {
         // Handle 404 status code
 }

src/Microsoft.Graph/Enums/GraphErrorCode.cs

@@ -80,6 +80,8 @@ public enum GraphErrorCode
         NameAlreadyExists,
         /// The action is not allowed by the system.
         NotAllowed,
+        /// The requested item is not not found.
+        NotFound,
         /// The request is not supported by the system.
         NotSupported,
         /// Parameter Exceeds Maximum Length.