Enforce supplying the export option to checkpoint command

[SergeyKanzhelev]

What happened:

Make troubleshooting of this: #1233 (comment) easier

What you expected to happen:

Export argument needs to be required and better documented as a tar file path.

How to reproduce it (as minimally and precisely as possible):

See #1233 (comment)

cc: @adrianreber

/help

[k8s-ci-robot]

@SergeyKanzhelev:
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In response to this:

What happened:

Make troubleshooting of this: #1233 (comment) easier

What you expected to happen:

Export argument needs to be required and better documented as a tar file path.

How to reproduce it (as minimally and precisely as possible):

See #1233 (comment)

cc: @adrianreber

/help

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[adrianreber]

I can look into this.