backups: improve error handling in Extract_Worker
backups: improve error handling in Extract_Worker

Terminate children when exception raised. Also rename tar2_command to
tar2_process to improve readability.

QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin