Separate out health check errors for Backends

[spencerhance]

Provides more specific errors in the logs

[k8s-ci-robot]

Hi @spencerhance. Thanks for your PR.

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[spencerhance]

Just saw #864 and rebased to only change one of the error messages

[freehan]

/ok-to-test

[freehan]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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