I used WaitUntilReady() in several stages, some of which fail to run under normal circumstances. For example, the SigningStage doesn't need to run if there are no signing requests for the board.

However, if WaitUntilReady returns false, a Failed result is recorded. Instead, we should SKIP the stage.


Discovered because of adityakali@google.com:

> Though it seems the lakitu_next-release builder started failing
> https://uberchromegw.corp.google.com/i/chromeos/builders/lakitu_next-release
> and the reason is unclear. Actually, only the 'cbuildbot' step is
>  marked as failed, but the image build and test and all other steps are 
> successful.