DAST fails when the ZAP server has shutdown

What does this MR do?

If DAST attempts to shutdown the ZAP server at the end of a scan and the server is not running, then it can be concluded that something went wrong during the scan. An example of this is when ZAP halts because of lack of memory.

Previously, DAST would exit with an exit code of zero. This MR ensures that an error is raised, which causes DAST to exit with a non-zero exit code.

What are the relevant issue numbers?

gitlab-org/gitlab#232648 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer