Low: mainloop: Suppress scary looking error message that isn't actually a problem

There's a race condition where we might try and cancel an operation
at the same time it is completing. Depending on the order mainloop
interprets those to conditions, this error message might show up.
The message makes it look like something terrible happend, an operation
failed to stop... oh no!  In reality, the operation already stopped on its
own and we detect that right after this error message. The only time
this error actually means something to us is if the operation timesout,
we never get the SIGCHLD signal, and we call kill again on the pid which
fails.