Add and emit download action error codes.

BUG=560
TEST=unit tests

Review URL: http://codereview.chromium.org/3046007
diff --git a/action_processor.h b/action_processor.h
index f04345d..7b1a312 100644
--- a/action_processor.h
+++ b/action_processor.h
@@ -25,6 +25,10 @@
 enum ActionExitCode {
   kActionCodeSuccess = 0,
   kActionCodeError = 1,
+  kActionCodeInstallDeviceOpenError = 2,
+  kActionCodeKernelDeviceOpenError = 3,
+  kActionCodeDownloadTransferError = 4,
+  kActionCodeDownloadHashMismatchError = 5,
 };
 
 class AbstractAction;