Better diagnostics when an SDL_GPU backend fails to initialize #745

Job	Run time
FreeBSD	1m 15s
iOS/tvOS SDL3 (iOS, iphoneos, arm64)	47s
iOS/tvOS SDL3 (iOS, iphonesimulator, x86_64;arm64)	1m 22s
Rocky Linux MinGW SDL3	1m 6s
iOS/tvOS SDL3 (tvOS, appletvos, arm64)	45s
Rocky Linux MinGW	1m 7s
Rocky Linux	1m 46s
iOS/tvOS SDL3 (tvOS, appletvsimulator, x86_64;arm64)	1m 7s
Debian Linux PPC	1m 48s
Debian Linux PPC SDL3	1m 4s
Rocky Linux SDL3	1m 28s
macOS (x86_64)	43s
macOS (Universal) SDL3	1m 16s
Windows (MSVC)	1m 27s
	17m 1s

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