RSVP.reject where the arg is not of type Error breaks error dispatching

[jasonmit]

The following will throw with Cannot read property 'indexOf' of undefined

run(RSVP, 'reject', 'foo');

(

ember.js/packages/ember-metal/lib/error_handler.js

Lines 5 to 14 in 39b606f

	let getStack = function(error) {
	var stack = error.stack;
	var message = error.message;
	if (stack.indexOf(message) === -1) {
	stack = message + '\n' + stack;
	}
	return stack;
	};

) always assumes what it received is it an instance of an Error and has a stack.

This is currently happening on ember-alpha, passes on current stable release.

Fail test: jasonmit@54d1665