Fixed|libdeng2|FileSystem: Creating native directories when inheriting feeds

If a DirectoryFeed is allowed to create native directories, a subfeed can
always be created.

Also improved an exception message when failing to create a new file.
Tests|glsandbox: Testing different Bank configs

There are various ways to use a Bank:
1) Non-threaded, set image in notification.
2) Non-threaded, use synchronous data access.
3) Threaded, set image in notification.
4) Threaded, use synchronous data access.