Emit an error if no package.json is found

[Tom Rochette]

Hello, I spent a couple of minutes trying to figure out why no package was being built only to discover that if no package.json is part of the src stream nothing would be built.

I think it would make sense to notify the user that a package.json is expected but was not found.

[Bas Zalmstra]

Thanks, this issue saved me a lot of trouble.