Fix panic when cache file can't be Stat-ed

[sarahhodne]

The code currently assumes that Stat always return a FileInfo object,
but if the file can't be Stat-ed (e.g. due to a permissions error), Stat
can return a nil FileInfo, plus an error. The error is only handled if
it's a "not exists" error, but there are other errors that can be
returned too.

This wasn't caught by the tests, since the mocked out Stat always
returns a FileInfo, even when an error is returned.

This commit also changes from os.IsNotExist(err) to errors.Is(err, fs.ErrNotExist) as recommended by the documentation for os.IsNotExist.

Fixes #349.

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[k8s-ci-robot]

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[nckturner]

/retest

[nckturner]

Thanks @sarahhodne, nice fix.

[sarahhodne]

The test failure doesn't seem related to my change, and I can't seem to reproduce it locally, so I think it might be a flake.

/retest

[k8s-ci-robot]

@sarahhodne: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

The test failure doesn't seem related to my change, and I can't seem to reproduce it locally, so I think it might be a flake.

/retest

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[nckturner]

/ok-to-test

[nckturner]

Agreed. If it fails again I'll take a closer look.

[sarahhodne]

Looks like it was a flake, so opened a ticket to track that, since I think the bot said that in the last message (though that appears to have been deleted now): #413

[nckturner]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nckturner, sarahhodne

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