Raise error at slice ndarray interface

On Mon, Sep 01, 2014 at 04:40:07AM -0700, pbrunet wrote:
> I really don't like testing we are bad and be sure we will stay in this bad
> state. I would prefer adding an issue about it in the bug tracker until it is
> really fixed.

Failing gracefully instead of yielding an error is an import part of the
UI.

The piece of code you're adding just shows this. And every piece of code
should be tested.

I cannot find the link to the talk from the sqlite developer, that says
he's testing correct error raising as well as correct processing, but it
was a really interesting one!