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.
You need a default case that does nothing, otherwise this will throw MatchError, see: https://travis-ci.org/sbt/sbt/jobs/38639951#L4395