7824: Several improvements:

7824: Several improvements:
* Added error reporting on cases where the user ask for downloading a nonexisting file/subdir inside a collection.
* Enhanced error message when the user ask to download a non existing collection.
* Improved the collection traversal when downloading a subcollection.
* Exit with an error message when asking for a subcollection name without the trailing '/'.
* Error out if asking to get a subcollection with '-' as destination.
* Test updated.