Improve stacktrace for failed dependency loading #397

Originally posed by @OliverJAsh in systemjs/systemjs#512:

This is what the errors looks like when a dependency fails to load:

Potentially unhandled rejection [3] Internal Server Error: <dependency>
    Error loading <dependency> from <requester>
    Error loading <dependency> (WARNING: non-Error used)

Sometimes dependency is a nested dependency of a dependency of requester, e.g. requester -> foo -> bar -> dependency. Is it possible to show the immediate requester (bar)?