Fail if envsubst does not find matching variables

[raffis]

Description

Currently the envsubst replaces the the placeholders with an empty string if no matching variable is found. This can lead to very odd problems. I know you could set a default value but that doesn't really solve the problem.

Proposal

There should be a way to let the entire kustomization fail if no such variable exists. I reckon this should even be the default behavior. As far as I checked drone/envsubst doesn't really support this but maybe swap the implementation?