Change missing cache directory error to warning

[aparnajyothi-y]

Description:
This PR updates the cache lookup logic to log a warning instead of throwing an error when a cache folder path is not found on disk.

Previously: action failed with Error: Cache folder path ... doesn't exist.

Now: action logs a warning via core.warning() and continues execution.

Motivation: In many cases, missing cache simply means there are no dependencies to cache, which should not fail the workflow.

This improves developer experience by preventing unnecessary workflow failures while still surfacing useful diagnostic information.

Related issue:
#436

[Copilot]

Pull Request Overview

This PR changes the error handling behavior when cache directories don't exist during the cache save operation. Instead of throwing an error that would fail the workflow, the action now logs a warning and continues execution.

• Replaces error throwing with warning logging when cache directories are missing
• Adds early return to prevent further cache operations when directories don't exist
• Improves developer experience by avoiding unnecessary workflow failures

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[simondeziel]

Thanks, LGTM!