This PR should fix #626

Described issue probably occurs in multi-threaded environment because the reported error throws FileAlreadyExistsException even though we checked first that file doesn't exist.

As per documentation of java.nio.file.Files#createDirectory:

Creates a new directory. The check for the existence of the file and the creation of the directory if it does not exist are a single operation that is atomic with respect to all other filesystem activities that might affect the directory

Since we only care that directory with the given name exists, we can just catch and ignore FileAlreadyExistsException in this case.