FrameworkDetector swallows the stderr output of the JS env

[sjrd]

This makes it very hard to understand and fix issues happening at loading time--typically of jsDependencies or from the set-up code of JS envs themselves, such as require("jsdom")--as all we get is:

> testingExample/test
[trace] Stack trace suppressed: run last testingExample/test:loadedTestFrameworks for the full output.
[error] (testingExample/test:loadedTestFrameworks) org.scalajs.jsenv.ExternalJSEnv$NonZeroExitException: Node.js with JSDOM exited with code 1
[error] Total time: 2 s, completed 17-sept.-2016 14:36:40