Log warning when invalid dir passed to kubectl proxy --www

[CaoShuFeng]

Release note:

Log warning when invalid directory is passed to `kubectl proxy --www`

[k8s-ci-robot]

Hi @CaoShuFeng. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with @k8s-bot 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.

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. I understand the commands that are listed here.

[k8s-reviewable]

This change is 

[soltysh]

/lgtm
@k8s-bot ok to test
@CaoShuFeng mind updating release notes, this is affecting output

[CaoShuFeng]

Release note is added.

[soltysh]

Awesome, thanks! Now all we need is an approval from @deads2k since bot picked him :)

[deads2k]

/approve

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CaoShuFeng, deads2k, soltysh

Needs approval from an approver in each of these OWNERS Files:

• pkg/kubectl/OWNERS [deads2k]

You can indicate your approval by writing /approve in a comment
You can cancel your approval by writing /approve cancel in a comment

[k8s-github-robot]

Automatic merge from submit-queue