fix: add check before performing chmod on the code coverage path

[QuakeEye]

Add check before performing chmod on the code coverage path, as it does not exist if the Code Coverage package is not installed, or is not set up correctly.

This was previously not done, causing me to spend a lot of time on trying to figure out why the action did not function as I expected it to.
I have also added a general message that gives some hints as to why the code coverage retrieval might not work.

Changes

• Added a check before performing chmod on the expected code coverage report path
• Added a general message to guide users when the path does not exist

Related Issues

• BUG: Test-runner actions fails with a "chmod: cannot access '/github/workspace/CodeCoverage': No such file or directory" error #218
• chmod: cannot access '/github/workspace/CodeCoverage' #260

Related PRs

• none

Successful Workflow Run Link

PRs don't have access to secrets so you will need to provide a link to a successful run
of the workflows from your own repo.

• https://github.com/QuakeEye/unity-test-runner/actions/runs/8080679270

Checklist

• Read the contribution guide and accept the code of conduct
• Docs (If new inputs or outputs have been added or changes to behavior that should be documented. Please make a PR
  in the documentation repo)
• Readme (updated or not needed)
• Tests (added, updated or not needed)

[github-actions]

[webbertakken]

Excellent!