MSFT_texture_dds: behaviour for unreachable or incorrect dds files

[andreasplesch]

Saw #1359 and see there is likely little motivation to maintain the MSFT_texture_dds extension but implementing requires addressing the case where the extension is supported but the .dds file itself is not available or not conforming or otherwise not loadable.

Is there a preference if in this case the loader should fall back to the regular texture source (likely) or fail (perhaps with a black texture) ?

In general, it would be helpful if the extension would follow the webp texture extension template for well defined behavior. I think we will end up doing this.