convert all warnings into errors

This makes the previous warnings errors and it will fail
    the shrinkwrap command instead of continuating.

This means that if any npm-shrinkwrap.json file fails
    validation the author is now forced to fix it
    immediately instead of recommended to fix it.

We've seen that the recommendations are completely
    ignored so we have to enforce this.

cc @sh1mmer @jsu1212 @Matt-Esch @kriskowal
Do you also want to include warnings here?  Might be useful to print both warnings and errors in the same output?  Or maybe you have to correct errors first before you even get warnings?