Rename ActionExitCode to ErrorCode

Nowadays ActionExitCode is used throughout the codebase so use a more
generic name to reflect this.

BUG=chromium:216507
TEST=unit tests pass

Change-Id: I23d1d7e2676443251dbc42ed137fd018aadfa8a3
Reviewed-on: https://gerrit.chromium.org/gerrit/49512
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/download_action.h b/download_action.h
index 6e334f6..318ece8 100644
--- a/download_action.h
+++ b/download_action.h
@@ -114,7 +114,7 @@
 
   // Used by TransferTerminated to figure if this action terminated itself or
   // was terminated by the action processor.
-  ActionExitCode code_;
+  ErrorCode code_;
 
   // For reporting status to outsiders
   DownloadActionDelegate* delegate_;