Fix for bug 769.

Fixed incorrect log message which indicates failure of flow uninstallation.

Change-Id: I728ac6ec77acd13f1becdf12f03e39b8af641e77
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>
(cherry picked from commit 07bbee320f3c15468e4b0e3adfec2e68628edacb)