Fix error code for no update.

PayloadState does not get notified if there's no update, so the error
code will not be updated, move this to UpdateAttempter.

We were using the generic kError for no update, this change added a
new error code kNoUpdate for this, now update_engine_client will show
kNoUpdate instead of kSuccess or a random last error.

Bug: 36946608
Test: update_engine_client --check_for_update --follow
Change-Id: Ie1e94841788437140e0894dc2e73c314a7564108
(cherry picked from commit 89e24c11b406fe048382bdf1c89334e10417b899)
Reviewed-on: https://chromium-review.googlesource.com/1055857
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>
diff --git a/common/error_code_utils.cc b/common/error_code_utils.cc
index 2c485a2..6b72eee 100644
--- a/common/error_code_utils.cc
+++ b/common/error_code_utils.cc
@@ -148,6 +148,8 @@
       return "ErrorCode::kOmahaUpdateIgnoredOverCellular";
     case ErrorCode::kUpdatedButNotActive:
       return "ErrorCode::kUpdatedButNotActive";
+    case ErrorCode::kNoUpdate:
+      return "ErrorCode::kNoUpdate";
       // Don't add a default case to let the compiler warn about newly added
       // error codes which should be added here.
   }