Replace boolean fullscreen error with more descriptive strings

This implements a TODO to expand the error code to be more than just
"fullscreen error". No functional change, other than to issue better
error messages.

Bug: 1307772
Change-Id: I6e00871effe999ab3265319422f64a9bc609ca96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4018694
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1069765}