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
It could be helpful to include the cookbook path in this error. It's possible the user has multiple configs and they're not sure which one they're getting. It'll save some thought if the error points out where it's looking.