[BUGFIX] Fix file and folder action checks if file and/or folder exists

[djcprinse]

Some methods needs a separate check if the file or folder exists.

For example: The resource storage now fails on the addFile method because this throws an exception if the file already exists. Even when the DuplicationBehaviour::REPLACE has been set in the resource storage.

The generic methods checkFileExists and checkFolderExists have been removed and in each action method the correct checks have been added.