Several parts of this function were incorrectly checking for errors or
passing the wrong parameters.  If screenshotting fails, throw an exception
in dEQP so that it is converted to a test failure instead of hanging the
test.

BUG=angleproject:2075

Change-Id: I19b55b273d0d3f1b47c1dac6affca69de8db21e0
Reviewed-on: https://chromium-review.googlesource.com/587300
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>