Missing results summary when test fails

When I run amber with multiple scripts, and one of them is unsupported, then I do not get a proper summary of the results:

```
$ amber draw_triangle_list_hlsl.amber user_type.amber
Unable to find Vulkan device supporting:
VK_GOOGLE_user_type
```

I would be nice to see something like:

```
Summary: 1 pass, 0 fail, 1 unsupported
```