Fix ktxTexture2_DecodeAstc error returns and document them. #1006

Job	Run time
windows (windows-latest, Visual Studio 17 2022, v143, x64, NO, Debug,Release, ON, ON, OpenGL+Vulk...	27m 15s
windows (windows-latest, Visual Studio 17 2022, v143, x64, NO, Release, OFF, OFF, OFF, OFF, OFF, ...	4m 26s
windows (windows-latest, Visual Studio 17 2022, v143, x64, NO, Release, OFF, OFF, OFF, OFF, OFF, ...	4m 46s
windows (windows-latest, Visual Studio 17 2022, v143, x64, NO, Release, OFF, OFF, OFF, OFF, OFF, ...	4m 13s
windows (windows-latest, Visual Studio 17 2022, CLangCL, x64, NO, Debug,Release, ON, ON, OpenGL+V...	24m 21s
windows (windows-latest, Visual Studio 17 2022, CLangCL, x64, NO, Release, OFF, OFF, OFF, OFF, OF...	4m 58s
windows (windows-latest, Visual Studio 17 2022, CLangCL, x64, NO, Release, OFF, OFF, OFF, OFF, OF...	5m 0s
windows (windows-latest, Visual Studio 17 2022, CLangCL, x64, NO, Release, OFF, OFF, OFF, OFF, OF...	4m 58s
windows (windows-2019, Visual Studio 16 2019, v142, x64, Debug,Release, ON, ON, OpenGL+Vulkan, ON...	25m 30s
windows (windows-latest, Visual Studio 17 2022, CLangCL, arm64, Debug,Release, ON, ON, OpenGL+Vul...	20m 27s
	2h 5m 54s

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