Add default messages for custom exceptions

Right now, our exception messages come from the API response in most
cases. But if for any reason there isn't one, we supply a default. I
realized these messages might make it to people's logs so it might be
nice to spend time later making them even more useful.

Octokit/Exceptions/ApiException.cs

@@ -36,7 +36,7 @@ namespace Octokit
 
         public override string Message
         {
-            get { return ApiErrorMessageSafe; }
+            get { return ApiErrorMessageSafe ?? "Request Forbidden"; }
         }
 
         public HttpStatusCode StatusCode { get; private set; }