verify-boilerplate: stricter error checking

[justinsb]

If python isn't found, without this fix the script exits with code 0,
despite not checking boilerplate.

[justinsb]

Figured I should probably fix the test also, not just break it, so version-bumped to python3 :-)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hakman, johngmyers

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[johngmyers]

/retest

[k8s-triage-robot]

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/retest

[justinsb]

/test pull-kops-build

[johngmyers]

/retest