I've been thinking the option --fail-level / -fl which is error by default, but could be changed to warning/warn. We've had similar request and they make sense: one should be able to execute a build without any warnings.
I did an attempt to hook into the Logger to but that seems very tricky. The other option is to have this level available in the API, so Maven core and plugins can verify it for themselves and decide to throw an Exception. It is a simple implementation, but now every component is responsible for checking it, not really my preferred solution.