Patch-package

Improve error messages

Reduce yarn error to a warning

- Require yarn as a peer dependency
- Remove support for NPM

Moving fast and breaking things. It turns out yarn doesn't run the prepare hook
after removing a package, so we use patch-package to patch a local version of
yarn. I'm not proud of this. Probably wouldn't have released this in the first
place if I had known that yarn didn't have all the right hooks. Oh well. Now I
have a reason to contribute to Yarn I guess.

- Fix patch creation logic around nested node_modules

- Enable picking up changes in nested node_modules folders.
- Enable adding new files, not just patching existing files.

- Fix bug that made exit code 1 regardless of success or failure.