Check if context.DistDir is undefined and warn that ember-cli-deploy-build is probably missing (improvement, usability)

[RobIsHere]

https://github.com/arenoir/ember-cli-deploy-rsync/blob/master/index.js#L32

Current error message:

- rsync: change_dir "/root/projects/xyz//undefined" failed: No such file or directory (2)
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1183) [sender=3.1.1]