screenshot layer does not report an error for a non-numeric Frames value

[lunarpapillo]

Cosmetic. First found during SDK testing: https://gitlab.khronos.org/vulkan/Vulkan-SDK-Packaging/-/issues/1546

When the screenshot layer is given a negative Frames value, it correctly provokes an error:

screenshot: Range error

But when it is given a non-numeric Frames value, no error is produced.

I expect an error similar to the Range error to be reported in this case.