Allow missing secrets.json, but warn in effect log

[roberth]

Especially since GitToken, the lack of a secrets.json does not have to be a configuration error.
Nonetheless, we'll want to encourage creation of the file, but not necessarily automate the creation, because that may interfere with some methods of secrets deployment to the agent machine.
Suggested implementation:
Add a warning without the location at the start of the effect log, and a warning with location in the local hercules-ci-agent.service log.