Port MpReturnCode

Might consider renaming it later. `Mp` is a bit weird, even though
it makes the name of the enum quite conveniently short.

Mediapipe.Net/Native/MpReturnCode.cs

@@ -0,0 +1,33 @@
+// Copyright (c) homuler and Vignette
+// This file is part of MediaPipe.NET.
+// MediaPipe.NET is licensed under the MIT License. See LICENSE for details.
+
+namespace Mediapipe.Net.Native
+{
+    /// <summary>
+    /// Code returned by most of MediaPipe's unsafe methods.
+    /// </summary>
+    public enum MpReturnCode : int
+    {
+        /// <summary>
+        /// Nothing gone wrong.
+        /// </summary>
+        Success = 0,
+        /// <summary>
+        /// A standard exception is thrown.
+        /// </summary>
+        StandardError = 1,
+        /// <summary>
+        /// Something other than standard exception is thrown.
+        /// </summary>
+        UnknownError = 70,
+        /// <summary>
+        /// SDK failed to set status code (bug).
+        /// </summary>
+        Unset = 128,
+        /// <summary>
+        /// Received SIGABRT.
+        /// </summary>
+        Aborted = 134,
+    }
+}