Detect yarn and surface correct error message

[elwayman02]

It would be nice to not have the error message for missing dependencies say to use npm install when you're using yarn in that project. Developers who are not familiar with what's going on will just trust the error message instead of realizing that they should actually have run yarn install.

[rwjblue]

I think this is entangled (somewhat) with #62.

[elwayman02]

yea, I agree, just wanted to file it as a separate issue since technically there may be a way to solve one without solving the other. In practice, though, it probably makes the most sense to resolve both issues at once.