refactor(eval error): modify error message to include its stack trace an...

...d throw it.

This change modifies the output from evaluation errors from

Error evaluating /home/brian/dev/day-calendar/jspm_packages/npm/react@0.12.0/addons.js

to

[ReferenceError: Error evaluating /home/brian/dev/day-calendar/jspm_packages/npm/react@0.12.0/addons.js
ReferenceError: navigator is not defined
    at evalmachine.<anonymous>:2:2270
    at Object.<anonymous> (evalmachine.<anonymous>:2:2899)
    at evalmachine.<anonymous>:4:4
    at doEval (/home/brian/dev/day-calendar/node_modules/systemjs/dist/system.src.js:2221:6)
    at __eval (/home/brian/dev/day-calendar/node_modules/systemjs/dist/system.src.js:2138:3)
    at Loader.exec [as __exec] (/home/brian/dev/day-calendar/node_modules/systemjs/dist/system.src.js:211:2)
    at load.metadata.execute (/home/brian/dev/day-calendar/node_modules/systemjs/dist/system.src.js:1092:10)
    at linkDynamicModule (/home/brian/dev/day-calendar/node_modules/systemjs/dist/system.src.js:541:29)
    at getModule (/home/brian/dev/day-calendar/node_modules/systemjs/dist/system.src.js:510:3)
    at /home/brian/dev/day-calendar/node_modules/systemjs/dist/system.src.js:545:10]


this allows quicker debugging of the evaluation error.
Right, I see, makes sense. It's old mistake of not throwing `Error`s. The stack is helpful for someone who wants to get into SystemJS and have the error caught by a debugger but it's not critical.