GN: Store parse error messages (so they aren't dropped due to races)

tools/gn/input_file_manager.cc

Index: tools/gn/input_file_manager.cc
diff --git a/tools/gn/input_file_manager.cc b/tools/gn/input_file_manager.cc
index 0b6159c92a507e601800fa472f1db88ae76ad6b9..c9e03f69bed55435e98c9463727e261e79c6d968 100644
--- a/tools/gn/input_file_manager.cc
+++ b/tools/gn/input_file_manager.cc
@@ -215,20 +215,12 @@ const ParseNode* InputFileManager::SyncLoadFile(
     }
   }
 
-  // The other load could have failed. In this case that error was probably
-  // printed to the console, but we need to return something here, so make up a
-  // dummy error.
-  //
-  // There is a race condition. The other load could have failed, but if the
-  // other thread is delayed for some reason, this thread could end up
-  // reporting the error to the scheduler first (since first error report
-  // wins). The user will see this one and the "real" one will be discarded.
-  if (!data->parsed_root) {
-    *err = Err(origin, "File parse failed.",
-        "If you see this, I'm really sorry, but a race condition has caused\n"
-        "me to eat your error message. It was crunchy. If the parse error\n"
-        "in your imported file isn't obvious, try re-running GN.");
-  }
+  // The other load could have failed. It is possible that this thread's error
+  // will be reported to the scheduler before the other thread's (and the first
+  // error reported "wins"). Forward the parse error from the other load for
+  // this thread so that the error message is useful.
+  if (!data->parsed_root)
+    *err = data->parse_error;
   return data->parsed_root.get();
 }
 
@@ -296,6 +288,8 @@ bool InputFileManager::LoadFile(const LocationRange& origin,
     if (success) {
       data->tokens.swap(tokens);
       data->parsed_root = root.Pass();
+    } else {
+      data->parse_error = *err;
     }
 
     // Unblock waiters on this event.