GN: Capture and print stderr from exec_script

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 34 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.h',
+        'exec_process.cc',
         'filesystem_utils.cc',
         'filesystem_utils.h',
         'functions_target.cc',
         'functions.cc',
         'functions.h',
         'function_exec_script.cc',
         'function_foreach.cc',
         'function_get_label_info.cc',
         'function_get_path_info.cc',
         'function_get_target_outputs.cc',
@@ skipping 128 matching lines @@
     {
       'target_name': 'gn_unittests',
       'type': '<(gtest_target_type)',
       '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 40 matching lines @@
       'type': 'executable',
       'sources': [
         'generate_test_gn_data.cc',
       ],
       'dependencies': [
         '../../base/base.gyp:base',
       ],
     }
   ],
 }