[Client] Fix possible goroutine leak.

Running goroutines with pullLayer are blocked to send error of a
pull operation. If we abort pulling without notify them about
cancelation they will get stucked forever. To avoid this possible
leak cancelCh was introduced. In case of abort we close that channel
to notify other goroutines about cancelation.
Typo fix
Merge pull request #766 from noxiouz/fix_possible_goroutine_leak

[Client] Fix possible goroutine leak in pull