Fix for https://github.com/sbt/sbt/issues/1648 - warn about multiple or no main classes

Works, but doesn't look like the best way to handle that.

My PR would benefit from some hints on how to handle logging in sbt itself.
Yeah, I think I'd like the above option better, or make "warnAboutNoMainClass as its own standalone method returning "Unit" and just call it from the right location.

In any case, generally LGTM, but we need you to rebase as we've drifted from master.