Optional msg argument to leiningen.core.main/exit. Use it for abort.

- New signature: leiningen.core.main/exit [exit-code & msg]
- msg used when *exit-process?* is false as the Exception message

The following issues have been created for projects which are (lein-junit, test2junit) or *might* (pallet/pallet, pallet/nrepl-main) be impacted by this change:

- https://github.com/febeling/lein-junit/issues/20
- https://github.com/ruedigergad/test2junit/issues/1
- https://github.com/pallet/pallet/issues/315
- https://github.com/pallet/nrepl-main/issues/1