Log errors from APIAvailability probes

[nojnhuh]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This PR adds a new log when requests against the API server's /readyz endpoint fail. Currently the only logs when that happen look like this:

cluster not available; HTTP status code: 0

This change will log any error that might have occurred for that request, like a TCP connection or DNS error.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: nojnhuh
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[nojnhuh]

Here's a concrete example of the error that gets logged:

W0710 03:20:23.091522   37631 api_availability_measurement.go:114] failed to reach cluster API server: an error on the server ("[+]ping ok\n[+]log ok\n[-]etcd failed: reason withheld\n[+]etcd-readiness ok\n[+]informer-sync ok\n[+]poststarthook/start-apiserver-admission-initializer ok\n[+]poststarthook/generic-apiserver-start-informers ok\n[+]poststarthook/priority-and-fairness-config-consumer ok\n[+]poststarthook/priority-and-fairness-filter ok\n[+]poststarthook/storage-object-count-tracker-hook ok\n[+]poststarthook/start-apiextensions-informers ok\n[+]poststarthook/start-apiextensions-controllers ok\n[+]poststarthook/crd-informer-synced ok\n[+]poststarthook/start-system-namespaces-controller ok\n[+]poststarthook/start-cluster-authentication-info-controller ok\n[+]poststarthook/start-kube-apiserver-identity-lease-controller ok\n[+]poststarthook/start-kube-apiserver-identity-lease-garbage-collector ok\n[+]poststarthook/start-legacy-token-tracking-controller ok\n[+]poststarthook/start-service-ip-repair-controllers ok\n[+]poststarthook/rbac/bootstrap-roles ok\n[+]poststarthook/scheduling/bootstrap-system-priority-classes ok\n[+]poststarthook/priority-and-fairness-config-producer ok\n[+]poststarthook/bootstrap-controller ok\n[+]poststarthook/start-kubernetes-service-cidr-controller ok\n[+]poststarthook/aggregator-reload-proxy-client-cert ok\n[+]poststarthook/start-kube-aggregator-informers ok\n[+]poststarthook/apiservice-status-local-available-controller ok\n[+]poststarthook/apiservice-status-remote-available-controller ok\n[+]poststarthook/apiservice-registration-controller ok\n[+]poststarthook/apiservice-discovery-controller ok\n[+]poststarthook/kube-apiserver-autoregistration ok\n[+]autoregister-completion ok\n[+]poststarthook/apiservice-openapi-controller ok\n[+]poststarthook/apiservice-openapiv3-controller ok\n[+]shutdown ok\nreadyz check failed") has prevented the request from succeeding

Prettier:

[+]ping ok
[+]log ok
[-]etcd failed: reason withheld
[+]etcd-readiness ok
[+]informer-sync ok
[+]poststarthook/start-apiserver-admission-initializer ok
[+]poststarthook/generic-apiserver-start-informers ok
[+]poststarthook/priority-and-fairness-config-consumer ok
[+]poststarthook/priority-and-fairness-filter ok
[+]poststarthook/storage-object-count-tracker-hook ok
[+]poststarthook/start-apiextensions-informers ok
[+]poststarthook/start-apiextensions-controllers ok
[+]poststarthook/crd-informer-synced ok
[+]poststarthook/start-system-namespaces-controller ok
[+]poststarthook/start-cluster-authentication-info-controller ok
[+]poststarthook/start-kube-apiserver-identity-lease-controller ok
[+]poststarthook/start-kube-apiserver-identity-lease-garbage-collector ok
[+]poststarthook/start-legacy-token-tracking-controller ok
[+]poststarthook/start-service-ip-repair-controllers ok
[+]poststarthook/rbac/bootstrap-roles ok
[+]poststarthook/scheduling/bootstrap-system-priority-classes ok
[+]poststarthook/priority-and-fairness-config-producer ok
[+]poststarthook/bootstrap-controller ok
[+]poststarthook/start-kubernetes-service-cidr-controller ok
[+]poststarthook/aggregator-reload-proxy-client-cert ok
[+]poststarthook/start-kube-aggregator-informers ok
[+]poststarthook/apiservice-status-local-available-controller ok
[+]poststarthook/apiservice-status-remote-available-controller ok
[+]poststarthook/apiservice-registration-controller ok
[+]poststarthook/apiservice-discovery-controller ok
[+]poststarthook/kube-apiserver-autoregistration ok
[+]autoregister-completion ok
[+]poststarthook/apiservice-openapi-controller ok
[+]poststarthook/apiservice-openapiv3-controller ok
[+]shutdown ok
readyz check failed