Prevent early exit from compress images script

What does this MR do and why?

Adjusts logic in compress-images.sh so that it doesn't exit with an error if it has an issue with a single image.

This is hopefully going to fix the failed 16.0 pipeline, see technical-writing#1065 (comment 2011655198).

Screenshots, screen recordings, or links to review app

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Run make compile to compile the site
3. Run the compress script: ./scripts/compress_images.sh public

Even if the script encounters an error on a single image, it will continue and exit 0.

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