Update the script to check coverage generation is enabled, and provide a sensible error message if it is not.
Change-Id: I42f2b97d18bb3be2d081200cb682ea310476943f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/70520
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: David Neto <dneto@google.com>