If an issuer is defined with wrong server endpoint, the error message should be more helpful

[MartinWeindel]

How to categorize this issue?

/area control-plane
/kind enhancement

What would you like to be added:
If the server endpoint is wrongly defined:
e.g.

    - type: shoot-cert-service
      providerConfig:
        apiVersion: service.cert.extensions.gardener.cloud/v1alpha1
        issuers:
          - name: letsencrypt
            email: ...
            server: https://acme-v02.api.letsencrypt.org/.   #wrong, must be: https://acme-v02.api.letsencrypt.org/directory

The shoot status only reports some generic error message without details.

Issuer "shoot--foo--bar/letsencrypt" is unhealthy: issuer state is "Error" ("Ready" expected)

The error message should have be more helpful.

[gardener-robot]

@MartinWeindel Label area/controlplane does not exist.