Fail a pipeline job when defined secrets do not exist in Vault

Problem

Currently if a secret defined under the secrets keyword does not exist in Vault the CI will not give any warning or error that that's the case but the environment-variable will bet set to <nil>. Problem appears in Gitlab: 14.5.4-ee.

Proposal

MVC

Job should fail with an error in the 'Resolving secrets' part of the CI-job logs when the defined secrets don't exist in Vault, to match the experience users have via CLI. See related comment

Error text:

Secrets defined at /some/secret/path/FOO do not exist in Vault.