doc: explain error message on missing main file

Added a hint saying that node uses the default "Cannot find module"
error when requiring a module for which the "main" file specified in
the package.json is missing.

PR-URL: https://github.com/nodejs/node/pull/5812


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>