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/TwoFactorRequiredExceptionTests.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Net;
+using System.Text;
+using System.Threading.Tasks;
+using Octokit.Internal;
+using Xunit;
+
+namespace Octokit.Tests.Exceptions
+{
+    public class TwoFactorRequiredExceptionTests
+    {
+        public class TheConstructor
+        {
+            [Fact]
+            public void SetsDefaultMessage()
+            {
+                var response = new ApiResponse<object>
+                {
+                    StatusCode = HttpStatusCode.Unauthorized
+                };
+
+                var exception = new TwoFactorRequiredException(response, TwoFactorType.Sms);
+
+                Assert.Equal("Two-factor authentication code is required", exception.Message);
+            }
+        }
+    }
+}