sketching out the exception necessary when raising specific merge exceptions

Octokit/Exceptions/PullRequestMismatchException.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace Octokit
+{
+    /// <summary>
+    /// Represents an error that occurs when the specified SHA
+    /// doesn't match the current pull request's HEAD
+    /// </summary>
+#if !NETFX_CORE
+    [Serializable]
+#endif
+    public class PullRequestMismatchException : Exception
+    {
+        public PullRequestMismatchException()
+            : base("The merge operation specified a SHA which didn't match " +
+                   "the SHA of the pull request's HEAD")
+        {
+        }
+
+        public PullRequestMismatchException(string message)
+            : base(message)
+        {
+        }
+
+
+        public PullRequestMismatchException(string message, Exception innerException)
+            : base(message, innerException)
+        {
+        }
+
+#if !NETFX_CORE
+        /// <summary>
+        /// Constructs an instance of PullRequestNotMergeableException.
+        /// </summary>
+        /// <param name="info">
+        /// The <see cref="SerializationInfo"/> that holds the
+        /// serialized object data about the exception being thrown.
+        /// </param>
+        /// <param name="context">
+        /// The <see cref="StreamingContext"/> that contains
+        /// contextual information about the source or destination.
+        /// </param>
+        protected PullRequestMismatchException(SerializationInfo info, StreamingContext context)
+            : base(info, context)
+        {
+        }
+#endif
+    }
+
+
+}