don't warn about "0 errors" before landing

[Archaeopteryx]

When https://phabricator.services.mozilla.com/D191007 was landed, it showed the following message before landing which needed be confirmed:

The code review bot found 0 errors which should be fixed to avoid backout and 4 warnings.

If there are 0 errors, the message should be less scary and only mention the warnings (which won't cause a backout).

The code lives here.