Make new merge exceptions inherit from 'Octokit.ApiException'

Affect 'Octokit.PullRequestNotMergeableException'
and 'Octokit.PullRequestMismatchException'

Octokit/Exceptions/PullRequestNotMergeableException.cs

@@ -1,4 +1,7 @@
 using System;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Net;
 using System.Runtime.Serialization;
 
 namespace Octokit
@@ -10,26 +13,39 @@ namespace Octokit
 #if !NETFX_CORE
     [Serializable]
 #endif
-    public class PullRequestNotMergeableException : Exception
+    [SuppressMessage("Microsoft.Design", "CA1032:ImplementStandardExceptionConstructors",
+        Justification = "These exceptions are specific to the GitHub API and not general purpose exceptions")]
+    public class PullRequestNotMergeableException : ApiException
     {
-        public PullRequestNotMergeableException()
-            : base("The pull request is not in a mergeable state")
+        /// <summary>
+        /// Constructs an instance of the <see cref="Octokit.PullRequestNotMergeableException"/> class.
+        /// </summary>
+        /// <param name="response">The HTTP payload from the server</param>
+        public PullRequestNotMergeableException(IResponse response) : this(response, null)
         {
         }
 
-        public PullRequestNotMergeableException(string message)
-            : base(message)
+        /// <summary>
+        /// Constructs an instance of the <see cref="Octokit.PullRequestNotMergeableException"/> class.
+        /// </summary>
+        /// <param name="response">The HTTP payload from the server</param>
+        /// <param name="innerException">The inner exception</param>
+        public PullRequestNotMergeableException(IResponse response, Exception innerException)
+            : base(response, innerException)
         {
+            Debug.Assert(response != null && response.StatusCode == HttpStatusCode.MethodNotAllowed,
+            "PullRequestNotMergeableException created with the wrong HTTP status code");
         }
 
-        public PullRequestNotMergeableException(string message, Exception innerException)
-            : base(message, innerException)
+        public override string Message
         {
+            //https://developer.github.com/v3/pulls/#response-if-merge-cannot-be-performed
+            get { return ApiErrorMessageSafe ?? "Pull Request is not mergeable"; }
         }
 
 #if !NETFX_CORE
         /// <summary>
-        /// Constructs an instance of PullRequestNotMergeableException.
+        /// Constructs an instance of <see cref="Octokit.PullRequestNotMergeableException"/>.
         /// </summary>
         /// <param name="info">
         /// The <see cref="SerializationInfo"/> that holds the