Improve error messages when check-example-showcase-patches-still-work CI job fails

[alice-i-cecile]

I was slightly worried about contributor experience here. A failure currently looks like:

• CI doesn't tell you which patch failed, so that needs to be cross-referenced
• CI stops after the first failure

But with this running on merge_group, I guess it's mostly maintainers having to deal with / at least explain this to contributors?

Still, would something like this be helpful? (or maybe just a set -x)

CODE=0
for patch in tools/example-showcase/*.patch; do
  git apply --ignore-whitespace $patch || { echo "::error::$patch failed to apply."; CODE=1; }
done
exit $CODE

(output using that code here: https://github.com/rparrett/bevy/actions/runs/8286681390/job/22677245376#step:7:14)

Originally posted by @rparrett in #12457 (comment)

[rparrett]

Someone picking this up might also want to address this comment while they're in there. #12457 (comment)