Adding error propagation for K8s client creation in KatibClient

[akirillov]

What this PR does / why we need it:
This PR adds error propagation for K8s client creation in katib_client.NewClient()

Which issue(s) this PR fixes
If the Client creation fails for some reason - the error is silently ignored and Katib UI backend starts normally. However, when a new experiment is submitted from the UI, it results in nil pointer dereference panic because the Client has not been initialized.

Special notes for your reviewer:
This PR doesn't change any image versions

Release note:

Fixed nil pointer dereference in `KatibClient`

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This change is 

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[k8s-ci-robot]

Hi @akirillov. Thanks for your PR.

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[akirillov]

@googlebot I signed it!

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[gaocegege]

/ok-to-test
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gaocegege

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[gaocegege]

Thanks for your contribution! 🎉 👍

[akirillov]

/retest

[akirillov]

thanks for the review, @gaocegege!