fail boilerplate check if non-compliant files are detected

[embik]

The current boilerplate.py script doesn't return an exit code to signal that the check failed, so the make verify command doesn't fail. It prints that files are missing boilerplate, but that is hidden in the prow job logs. The verify prow job succeeds though.

This PR updates the Python script to return exit code feedback so that make verify fails. I discovered a non-compliant file, pkg/common/spinner.go, which was contributed in #148 by @Bharadwajshivam28. I'm not 100% sure if this PR needs sign off by them (I assume the contribution is made under Apache license anyway).

[k8s-ci-robot]

Hi @embik. Thanks for your PR.

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[rjsadow]

/ok-to-test

[dims]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dims, embik

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