Add default error handler to remote promises (#6151)

* Add failing spec for unhandled main process exception

* Remove unused return

* Use let/const instead of var

* Add spec for unhandled rejection in renderer process

* Prevent unhandled rejection defaul

* Use once instead of on

* Add default fulfilled/rejection handler to promise

spec/fixtures/module/unhandled-rejection.js

@@ -0,0 +1,3 @@
+exports.reject = function () {
+  return Promise.reject(new Error('rejected'))
+}