vtkWeb: Teach launcher.py to wait for ready_line

Rather than just waiting the full timeout check the output of the
process for the ready_line ( which is configured in launcher.json ).
This should resolve timing issues related to server websockets not
being ready to receive client connections. Also in error paths rather
than return status code 200 (ok) return a reasonable error code.

Change-Id: I3cd99ab5758184002500e3b9adc6208d0fd736e0