Incorrect return code when you try to lint non-existent files

[Tattoo]

If one tries to lint non-existent files, rflint returns with the code 0.

The problematic line is shown here; I believe either ERROR or "other" count should be increased so later on when reporting rflint would also fail.

Return code of zero is problematic especially in the context of CI, where automation relies on error codes to know should we continue or not. In our case for example, we use linting as a quality gate for later phases. Since we have a lot of test cases, we try to figure out in CI specifically what tests should go through the pipeline. If we have a bug in this, rflint does not stop the pipeline which becomes a problem in later phases.