warn when a generator is found with no default defined

[travi]

it has been a while since i've built a new generator, so i overlooked the requirement to name the sub-generator that is intended to be the default, even if it is the only sub-generator, as app. i burned several evenings trying to sort out why the generator was not showing up in the list shown by the yo command.

finally i opened yeoman/yeoman#1640 and was given a few tips that led me to realize the mistake. before opening the issue above, i had run yo doctor several times, so if it had warned me that it was finding my generator but not showing it in the list because of a lack of defining a default, i would have realized and fixed my issue far more quickly.

if it matters, i expect that this would mostly be helpful only for npm linked generators since those are most likely work-in-progress.