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.
right. only now I noticed how are the 'integration' tests for sbt projects done. Is there a way to verify some log output in those 'scripted' tests ? I could add some test for that