Better error message when failing on unknown targets (#1313) #2099

Job	Run time
Test CUDA support	38s
Clippy	9s
Test build-std (aarch64-tvos)	2m 8s
Test wasm (wasm32-unknown-unknown)	19s
Test build-std (aarch64-tvos-sim)	1m 44s
MSRV (ubuntu-latest)	23s
Rustfmt	11s
MSRV (windows-latest)	41s
Test build-std (x86_64-tvos)	2m 45s
semver-checks	25s
MSRV (macos-latest)	31s
Test wasm32-wasip1-thread	38s
Test (stable)	1m 31s
Test (beta)	1m 32s
Test (nightly)	1m 17s
Test (linux32)	2m 35s
Test (macos)	5m 49s
Test (aarch64-macos)	1m 37s
Test (aarch64-ios)	1m 29s
Test (win32)	2m 48s
Test (win64)	2m 53s
Test (mingw32)	2m 59s
Test (mingw64)	3m 21s
Test (windows-2019)	2m 39s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	3m 47s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	3m 56s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	4m 50s
	53m 35s

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