Prevent local storage setup on non-mounted paths

Fixes #269

storage/basestorage.py

@@ -58,6 +58,10 @@ class BaseStorage(StoragePaths):
     """ Called to perform any storage system setup. """
     pass
 
+  def validate(self):
+    """ Called to perform any custom storage system validation. """
+    pass
+
   def get_direct_download_url(self, path, expires_in=60, requires_cors=False):
     return None