scheduler: Log errors when stopping jobs.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
host: Jobs can be stopped before they are started.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>

Conflicts:
	host/rpc.go
Merge pull request #485 from flynn/fix-job-stops

Fix job stops