Retry choose_test_namespace on exception

What does this MR do?

Fixes gitlab-org/quality/staging#42 (closed) by retrying in choose_test_namespace on any exception.

What are the relevant issue numbers?

Closes gitlab-org/quality/staging#42 (closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer