package loader could give better error messages

[andychu]

I created a new package.json for "Jesyll", with dependencies ["markdown", "jsontemplate"], since I know I'll depend on those.

I then ran the program, and it said "can't find jesyll". And I tried with -d and -v and couldn't really see what the issue is. I was stuck on this and trying to compare with other packages until I figured out it's probably those nonexistent dependencies.

A nicer error message would be "can't find jsontemplate declared in jesyll/package.json". -v or -d could list the paths being searched too, like PYTHONVERBOSE=1 does.