GN: Capture and print stderr from exec_script

tools/gn/BUILD.gn

 # Copyright (c) 2013 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.
 
 defines = [ "GN_BUILD" ]
 
 static_library("gn_lib") {
   sources = [
     "action_target_generator.cc",
     "action_target_generator.h",
@@ skipping 30 matching lines @@
     "config_values_generator.cc",
     "config_values_generator.h",
     "copy_target_generator.cc",
     "copy_target_generator.h",
     "deps_iterator.cc",
     "deps_iterator.h",
     "err.cc",
     "err.h",
     "escape.cc",
     "escape.h",
+    "exec_process.cc",
+    "exec_process.h",
     "filesystem_utils.cc",
     "filesystem_utils.h",
     "functions.cc",
     "functions.h",
     "functions_target.cc",
     "function_exec_script.cc",
     "function_foreach.cc",
     "function_get_label_info.cc",
     "function_get_path_info.cc",
     "function_get_target_outputs.cc",
@@ skipping 147 matching lines @@
 }
 
 test("gn_unittests") {
   sources = [
     "action_target_generator_unittest.cc",
     "builder_unittest.cc",
     "c_include_iterator_unittest.cc",
     "command_format_unittest.cc",
     "config_values_extractors_unittest.cc",
     "escape_unittest.cc",
+    "exec_process_unittest.cc",
     "filesystem_utils_unittest.cc",
     "function_foreach_unittest.cc",
     "function_get_label_info_unittest.cc",
     "function_get_path_info_unittest.cc",
     "function_get_target_outputs_unittest.cc",
     "function_process_file_template_unittest.cc",
     "function_rebase_path_unittest.cc",
     "function_write_file_unittest.cc",
     "functions_target_unittest.cc",
     "functions_unittest.cc",
@@ skipping 34 matching lines @@
     ":gn_lib",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 }
 
 executable("generate_test_gn_data") {
   sources = [ "generate_test_gn_data.cc" ]
   deps = [ "//base" ]
 }