rosie go: check suite is registered prior to running gui on it.

Following recent cylc changes to the way the gui is launched it is now silently failing in the background for unregistered suites when launched by rosie go as a result of the way we run it via popen.

This change checks to see if the suite to be run is registered and either:
1) launches the gui if it is registered
2) displays an error message if it isn't

Effectively closes #929

@benfitzpatrick - please review.