Update CreateILBSubnet() to catch 400s as well

[spencerhance]

We should also expect to see 400s when attempting to create the subnet after the first time.

[k8s-ci-robot]

Hi @spencerhance. Thanks for your PR.

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[spencerhance]

/assign @MrHohn

[MrHohn]

/ok-to-test

[MrHohn]

/lgtm
/approve

[spencerhance]

@MrHohn fixed the error, can you re-lgtm?

[MrHohn]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: MrHohn, spencerhance

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