Implement RepositoryExistsException

This exception is thrown when we try to create a repository but it
already exists on the server.

Octokit/Helpers/ApiErrorExtensions.cs

@@ -0,0 +1,16 @@
+using System.Linq;
+
+namespace Octokit
+{
+    internal static class ApiErrorExtensions
+    {
+        public static string FirstErrorMessageSafe(this ApiError apiError)
+        {
+            if (apiError == null) return null;
+            if (apiError.Errors == null) return null;
+            var firstError = apiError.Errors.FirstOrDefault();
+            if (firstError == null) return null;
+            return firstError.Message;
+        }
+    }
+}