Check license key in project map

related to https://github.com/technomancy/leiningen/issues/1419

have a couple of questions:
* What should be the correct error message? Right now I'm using `"Project map must have a :license field"`.
* Is it ok to throw an exception in this case or we need to use `abort-msg` or something like that?
* What do we need to check inside `:license` key? Presence of `:name`?