Exit build-script verify-source-code with non-zero exit code if files don’t match
Exit build-script verify-source-code with non-zero exit code if files don’t match

Previously, `build-script.py verify-source-code` would just print an error if the generated files didn’t match, but return with exit code 0, which didn’t fail Swift CI.

apple/swift-syntax,apple/swift-syntax,apple/swift-syntax