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; }

Octokit/Exceptions/ApiValidationException.cs

@@ -33,6 +33,11 @@ namespace Octokit
                 "ApiValidationException created with wrong status code");
         }
 
+        public override string Message
+        {
+            get { return ApiErrorMessageSafe ?? "Validation Failed"; }
+        }
+
 #if !NETFX_CORE
         protected ApiValidationException(SerializationInfo info, StreamingContext context)
             : base(info, context)

Octokit/Exceptions/LoginAttemptsExceededException.cs

@@ -20,6 +20,11 @@ namespace Octokit
         {
         }
 
+        public override string Message
+        {
+            get { return ApiErrorMessageSafe ?? "Maximum number of login attempts exceeded"; }
+        }
+
 #if !NETFX_CORE
         protected LoginAttemptsExceededException(SerializationInfo info, StreamingContext context)
             : base(info, context)

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)

Octokit/Exceptions/TwoFactorChallengeFailedException.cs

@@ -23,6 +23,11 @@ namespace Octokit
         {
         }
 
+        public override string Message
+        {
+            get { return "The two-factor authentication code supplied is not correct"; }
+        }
+
 #if !NETFX_CORE
         protected TwoFactorChallengeFailedException(SerializationInfo info, StreamingContext context)
             : base(info, context)

Octokit/Exceptions/TwoFactorRequiredException.cs

@@ -31,6 +31,11 @@ namespace Octokit
             TwoFactorType = twoFactorType;
         }
 
+        public override string Message
+        {
+            get { return ApiErrorMessageSafe ?? "Two-factor authentication code is required"; }
+        }
+
 #if !NETFX_CORE
         protected TwoFactorRequiredException(SerializationInfo info, StreamingContext context)
             : base(info, context)