[tetrapak] fixing stoping routine

Summary: All running tasks are not right killed, in an error case.

Test Plan:
tetrapak <task>, that throws an error.
Example 'tetrapak clean' on xray_web

Reviewers: aschultz, vtarasen, orixmann, nkalashnikov

Reviewed By: nkalashnikov

Subscribers: aschultz, druss, mkraemer

Differential Revision: https://review.tpip.net/D1790