added exit status steps to gvm install feature (fixes #223)

i came across this issue when trying to automate the gvm setup using salt.

if a candidate is already installed, the exit status is interpreted has a failure. the same happens if you try to install a candidate and automate the 'set as default' answer.

i've updated the gvm install feature with what i believe it should be the common sense exit status (but then again, i might be missing the big picture). anyway, the stakeholders for the gvm project are definitely technical people, so i think the exit status should be part of the specs.