Fix premature reporting of action when GPUs are not detected

Fix premature reporting of action when GPUs are not detected

The decision about what to do when GPUs could not be detected is a
responsbility taken at a higher level, so the detection handling code
should just report the facts.

Clarified some of the intended usage in the documentation

Fixes #2374

Change-Id: I63b569e053dd88d66351640019efc758e91bbdff