Whenever an external storage of type Local points at a non-existing
directory, process this as a StorageNotAvailable instead of returning
404.
This makes desktop clients ignore the folder instead of deleting it when
it becomes unavailable.
The code change was limited to external storages to avoid issues during
setup and with the default home storage.
Signed-off-by: Vincent Petry <pvince81@yahoo.fr>
8d1a3daa3f