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/RateLimitExceededException.cs

@@ -52,6 +52,13 @@ namespace Octokit
         /// </summary>
         public DateTimeOffset Reset { get; private set; }
 
+        // TODO: Might be nice to have this provide a more detailed message such as what the limit is,
+        // how many are remaining, and when it will reset. I'm too lazy to do it now.
+        public override string Message
+        {
+            get { return ApiErrorMessageSafe ?? "API Rate Limit exceeded"; }
+        }
+
 #if !NETFX_CORE
         protected RateLimitExceededException(SerializationInfo info, StreamingContext context)
             : base(info, context)