Throw a warning/error when target service doesn't exist

[rvem]

By default

vault-secrets.secrets.example = {};

Will create example-secrets.service and use it as a dependency for example.service. However, if example.service doesn't exist, vault-secrets will still create it. While the actual target service(s) will miss the example-secrets.service dependency.

IMO, vault-secrets should either throw a warning or an error at evaluation time in case some services from vaule-secrets.secrets.<name>.services don't exist in systemd.services.