cleanup logging, return errors from pkg/ code

[xrstf]

This PR does a few minor things:

• Code inside pkg/ does not call log.Fatal() anymore, but instead returns errors. Only code in cmd/ exits.
• Logging has been improved / harmonized:
  • Log messages are capitalized.
  • I removed some seemingly unneccessary empty lines.
  • When an error happens during a file download, stderr is captured and returned, aiding in debugging.
• The cleanup code is idempotent now and does not error out anymore if a resource is already gone. This allows the user to just re-run it "until it works".
• The spinner is not stopped twice anymore.

[k8s-ci-robot]

Hi @xrstf. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[dims]

/ok-to-test

[dims]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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