Add C API (#317)

* Add C API

* Add installation instructions for the C API

c_api/error_impl.cpp

@@ -0,0 +1,28 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD+Patents license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// Copyright 2004-present Facebook. All Rights Reserved.
+// -*- c++ -*-
+
+#include "error_c.h"
+#include "error_impl.h"
+#include "FaissException.h"
+#include <exception>
+
+thread_local std::exception_ptr faiss_last_exception;
+
+const char* faiss_get_last_error() {
+    if (faiss_last_exception) {
+        try {
+            std::rethrow_exception(faiss_last_exception);
+        } catch (std::exception& e) {
+            return e.what();
+        }
+    }
+    return nullptr;
+}