Add better error message for missing package.json

Hopefully this will enable users that are using npm for the very first time to help themselves.

We are having alot of these errors in the last weeks and this would help in 99% of the issues reported, eg.:

https://github.com/isaacs/npm/issues/3997
https://github.com/isaacs/npm/issues/4000
https://github.com/isaacs/npm/issues/3972

Feel free to change the message text!