[AudioPlayer, Gallery, VideoPlayer] Fix error message

The string concatanation was interpreted as:
('msg ' + error.stack) || error
But the desired outcome is:
'msg ' + (error.stack || error)

When error.stack was undefined the error message was lost and logging:
'msg undefined'

Change-Id: I7c58e21439c00a3328fc21e122f6d521c272f6bb
Reviewed-on: https://chromium-review.googlesource.com/c/1426201
Commit-Queue: Trent Apted <tapted@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624733}
3 files changed