cylc lint - ability to return non-zero code when warnings raised?

[ColemanTom]

Problem

cylc lint always returns a 0 return code. When running a linter using pre-commit or via CI pipelines, cylc lint would hopefully return a non-zero return code so the linting will fail and raise an alert. To work around this, we could capture stdout and check for any error messages, and if there are, print them out and fail, but it's not optimal.

Proposed Solution

Add a flag to the CLI to fail on warning.