Warn user when 404 on loading a bower.json

It's possible that loading a bower.json could cause a 404 in the case
where the package doesn't include it (I've seen this happen).  It
doesn't prevent the rest of the modules from loading, but we should let
the user know of the problem. Fixes #4
Merge pull request #6 from bitovi/404

Warn user when 404 on loading a bower.json