If there's nothing on the screen, screenshot.py will throw an exception like:
File "/usr/local/autotest/bin/screenshot.py", line 21, in <module>
    image = crtcScreenshot(args.crtc)
  File "/usr/local/autotest/cros/graphics/drm.py", line 687, in crtcScreenshot
    "Unable to take screenshot. There may not be anything on the screen.")
RuntimeError: Unable to take screenshot. There may not be anything on the screen.

This can occur during a test failure, and this failure can suppress other errors, so we should eat it.