Image Capture layouttests: correct mockImageCaptureReady error callback

This CL corrects the uses of mockImageCaptureReady error path:
mockImageCaptureReady pings back an error function and doesn't reject
a Promise; also there's some minor editing to homogenise logs.

BUG=700607

Review-Url: https://codereview.chromium.org/2762883002
Cr-Commit-Position: refs/heads/master@{#458474}
5 files changed