Better error message when failing on unknown targets #2098

Job	Run time
Test wasm (wasm32-unknown-unknown)	50s
Test (stable)	3m 21s
Test (beta)	1m 35s
Test CUDA support	42s
Test build-std (aarch64-tvos)	2m 28s
Test (nightly)	1m 18s
Test wasm32-wasip1-thread	54s
Test build-std (aarch64-tvos-sim)	1m 43s
Test (linux32)	2m 2s
Test build-std (x86_64-tvos)	1m 42s
Test (macos)	4m 39s
Test (aarch64-macos)	1m 54s
Clippy	18s
Test (aarch64-ios)	1m 21s
MSRV (ubuntu-latest)	29s
Test (win32)	2m 27s
MSRV (windows-latest)	38s
Rustfmt	30s
MSRV (macos-latest)	31s
Test (win64)	2m 37s
Test (mingw32)	2m 46s
Test (mingw64)	3m 18s
semver-checks	17s
Test (windows-2019)	3m 29s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	3m 0s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	6m 4s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	4m 17s
	55m 10s

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