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
Merge remote-tracking branch 'laurentpetit/master'

Conflicts:
	leiningen-core/src/leiningen/core/main.clj
Make repl tests resilient to having .nrepl-port file in pwd.