flow_manager: fix two FlowManagerError format strings

BUG=None
Test=Call StopCapturingAudio or StopPlayingAudio with an invalid port

Change-Id: Id0b396c9572cde14e170c0c1950613dda24333a9
Reviewed-on: https://chromium-review.googlesource.com/1561230
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
1 file changed