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.Tests/Exceptions/RateLimitExceededExceptionTests.cs

@@ -28,6 +28,8 @@ namespace Octokit.Tests.Exceptions
                     "Mon 01 Jul 2013 5:47:53 PM -00:00",
                     "ddd dd MMM yyyy h:mm:ss tt zzz",
                     CultureInfo.InvariantCulture);
+                
+                Assert.Equal("API Rate Limit exceeded", exception.Message);
                 Assert.Equal(expectedReset, exception.Reset);
             }