Improve StaticWeb 404 on web-listings/index

A common scenario is for users to make a container public via an ACL.
They can then use a browser to display the objects in the container.
Next, they enable StaticWeb with something such as
"X-Container-Meta-Web-Index:index.html" -- but then get confused
because they get 404 Not Found (when index.html does not exist).

For someone who understands what they are doing, this makes sense.
However, we've had several customer escalations because of this.
Usually, they are just playing with our GUI and have no intention
of using StaticWeb for real -- the 404 looks like something is
broken; not the correct response.

The solution is to provide a better error message. This message is only
shown when StaticWeb is refusing to give a listing -- other 404 situations
are not affected. Also, a custom 404 error page is not affected.

Change-Id: I3ba8c48e0ce148d8e91a1e0dc16a0d37a692a24e