IOCTL: Warn instead of error at transaction cancel fail.

It's harmless to fail a transaction cancellation. It merely means either
the ID is not found, or the transaction has already been executed.
Downgrade log message from error to warning.

Signed-off-by: Edmond Chung <edmondchung@google.com>
Change-Id: Icac7fef1fafa723255de86e4ace597fd6f58a264
(cherry picked from commit b1d1afbcc81d46e0a85b343ff9f62d6f465f93b1)