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/TwoFactorChallengeFailedException.cs

@@ -0,0 +1,18 @@
+using Xunit;
+
+namespace Octokit.Tests.Exceptions
+{
+    public class TwoFactorChallengeFailedExceptionTests
+    {
+        public class TheConstructor
+        {
+            [Fact]
+            public void SetsDefaultMessage()
+            {
+                var exception = new TwoFactorChallengeFailedException();
+
+                Assert.Equal("The two-factor authentication code supplied is not correct", exception.Message);
+            }
+        }
+    }
+}