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.
So, we can't actually change the name of this method without breaking binary compatibility.  I agree that your name is better, so you can keep the old one and deprecate it if you want.