Local storage: Always try to create directories, handle already existing

This reduces the chances of races where two client try to upload the same file / two files with the same directory.

Not sure if this is the right way (or if the `.directory?` check should be left in); please advise.
This should probably rescue more narrowly (ideally JUST the directory already exists error). I just worry that otherwise this might obfuscate other sorts of errors (like permissions or something for instance).