Error out if an installation results in invalid peer dependencies.

Fixes #3079. Please also see #3102 and #3104. Maybe #3101 while you're at it.

Result is like this:

```
npm ERR! peerinvalid The package flatiron does not satisfy its siblings' peerDependencies requirements!
npm ERR! peerinvalid Peer flatiron-cli-config@0.1.3 wants flatiron@~0.1.9
npm ERR! peerinvalid Peer flatiron-cli-users@0.1.4 wants flatiron@~0.3.0

npm ERR! System Windows_NT 6.2.9200
npm ERR! command "node" "C:\\Users\\Domenic\\Dropbox\\Programming\\GitHub\\npm\\cli.js" "install" "jitsu"
npm ERR! cwd C:\Users\Domenic\Dropbox\Programming\GitHub
npm ERR! node -v v0.8.18
npm ERR! npm -v 1.2.3
npm ERR! code EPEERINVALID
```