Patch release v1.4.3 - Make cv2 install optional

As per discussion: vllm-project/vllm#8650 make cv2 optional.

What's Changed

• [Error Message] Improve error message by @patrickvonplaten in #54
• Make cv2 optional by @patrickvonplaten in #56

Full Changelog: v1.4.2...v1.4.3