Do not assert if process survives kill.

Summary:
If process survives kill, an error message is already printed to
indicate the problem. Since we cannot hope to kill the process,
gracefully handle this case.

BUG: 365766

Test Plan: Manual testing.

Reviewers: #kdevelop, brauch

Reviewed By: #kdevelop, brauch

Subscribers: brauch, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2195