GN: Capture and print stderr from exec_script

tools/gn/exec_process.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <string>
+
+namespace base {
+class CommandLine;
+class FilePath;
+}
+
+namespace internal {
+bool ExecProcess(const base::CommandLine& cmdline,
+                 const base::FilePath& startup_dir,
+                 std::string* std_out,
+                 std::string* std_err,
+                 int* exit_code);
+}  // namespace internal