cros_generate_update_payload: Make umount results more verbose.

Log more information about cleanup failures in cros_generate_update_payload.
This is being done to track down why mount points sometimes can't be removed
on machines with very heavy loads.

BUG=chromium:353558
TEST=Trybot testing (which generates payloads for tests).

Change-Id: Id1cc3e3f42a31dd664d0ca3a3e655dab32e7a630
Reviewed-on: https://chromium-review.googlesource.com/191305
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
1 file changed