Improve companion spawner error messages
Improve companion spawner error messages

Summary: At the moment our `JSONDecodeError` error messages are pretty useless. Typical the companion has crashed, so we really need to include its stderr as well.

Reviewed By: bestander

Differential Revision: D20138766

fbshipit-source-id: 3b51a99577c94128707092bc0bb3151aa37bb657

facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl