Generator lookup bug

RE: https://github.com/yeoman/yeoman/issues/1113

One of yoyo's internal methods, `_findGenerators`, is used to find a user's installed generators' package.json files. I didn't write it carefully enough to handle the case where it couldn't find it ( :( ). The diff will show pretty clearly where the bug was, and this commit should be all it takes to fix it up. `generator.resolved` is the entire path to the `.js` file that contains the generator's function definition.

I believe the type of users affected by this are:

- someone with custom generators without package.json files
- someone who has an installed generator with a base folder name that doesn't follow the format of `*generator*` (`generator-*` is caught, (e.g. `generator-angular`))

Is there anything else I can add to this function to secure it even more?