Print non-message to STDERR to avoid confusing emacs.

[ikirill]

This isn't a real issue at all, just a nitpick, but I'm using STDOUT to communicate with a lint process, and printing non-json messages that aren't real lint messages makes the communication a little messier to parse—the output is no longer even valid json.

[coveralls]

Coverage decreased (-0.7%) to 90.061% when pulling ecbb29a on ikirill:fix-println-stderr into 7f311ed on tonyhffong:master.

[TotalVerb]

LGTM. I think there are other places where we're printing to STDOUT too, but should be printing to STDERR, as noted here, but that can be fixed later. Tell me if you'd like a new tag — otherwise I'll hold off until after n-pass linting is done, as the current master temporarily has a lot more false positives than the latest release.

[TotalVerb]

Since we'll need #224 anyway, I've decided to just make a backport 0.5.1 release, and we may as well include this fix.