Remove try catch that causes DAST to succeed when the scan fails

What does this MR do?

Removes a try/catch statement that catches IOError, ScanNotStartedException and NoUrlsException from the ZAP scripts. When any of these errors occur, DAST exits with an exit code of zero, implying that the scan was successful.

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