Exit 1 when changes are made

[Almenon]

for use in CI it should exit 1 when changes are made, so people know they should make changes. They might not install the precommit hook. This would also match behavior of other linting tools

[feluelle]

It is more of a formatter than a linter. Ideally, there is no user action if you use -a. If you don't use it, I agree it should exit 1.