Issue 2391 - Check cookbooks exist in path(s) before attempting to upload them with --all

Made this change in response to #2391

* Added a check to make sure cookbooks exist in the path(s) before attempting to upload them all
* Added warning if no cookbooks exist
* Added tests for new logic
I realized it already existed, but this variable is kind of unfortunately named as it's not very specific. This line would be more immediately obvious if it was something like `if cookbooks_to_upload.any?`