Add FILES_CHECKER_FAILURE_LEVEL to support warnings instead of errors

[jspiro]

Description

Changes proposed in this pull request:

• These failures cause no real issues, they're just indicative of potential misconfigurations and/or are purely informative
• This allows the tool to report results while not failing

Related issue(s)

[jspiro]

Eh, I'm unsure if I still think this is valuable.

The use case is to permit preemptive patterns that may appear at some point in the future, like Dockerfile.

So if it's a warning, then it's hidden if you make the failure level error. It's just a record. This is fine.

But then when someone unintentionally misspells Jenkinfile instead of Jenkinsfile there is no way to differentiate it from Dockerfile – which is preemptive, which is an error?

So, I'm thinking the best to do is comment out preemptive entries, and enable NOT_OWNED_CHECKER_SKIP_PATTERNS="*" – this combination ensures any new files that don't have an owner get discovered, so a preemptive file would cause an error, then you'd see it commented out, and you'd uncomment it.

[jspiro]

Closing unless @mszostok wants it anyway.