Warn user when 404 on loading a bower.json
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

stealjs/system-npm,stealjs/system-json,stealjs/system-json,stealjs/steal-npm,stealjs/steal-npm,stealjs/system-npm