[Tech Debt] [Observability] Improve cluster-network-operator's status reporting for invalid proxy config

As an administrator, I specified proxy config that failed cluster-network-operator's validation, and I want CNO's "Degraded" status condition message to direct me to the configmap where it copied the previous, valid config, so that I can refer to the previous config as a reference for fixing up my broken config.

Currently, the status condition reports the following:

The configuration is invalid for proxy 'cluster' (failed to validate configmap reference for proxy trustedCA 'user-ca-bundle': failed to get trustedCA configmap for proxy cluster: ConfigMap "user-ca-bundle" not found). Use 'oc edit proxy.config.openshift.io cluster' to fix.

This message should include a statement like the following:

We'll continue to use our saved version while you fix this, so this is not currently an emergency, unless for some reason the old settings stop working.  Refer to the "trusted-ca-bundle" configmap in the "openshift-config-managed" namespace for the configuration that is currently in effect.

See https://coreos.slack.com/archives/CDCP2LA9L/p1616610130344000 for the origin of this story.