feat(cli): warn when removing the last/host target for a toolchain #56

Job	Run time
build-freebsd	32m 17s
build-linux-master	0s
build-linux-pr (dev, x86_64-unknown-linux-gnu)	7m 33s
build-all-features (x86_64-unknown-linux-gnu)	3m 24s
build-linux-pr (dev, armv7-unknown-linux-gnueabihf)	58s
build-linux-pr (dev, aarch64-linux-android)	1m 7s
build-linux-pr (release, x86_64-unknown-linux-gnu)	3m 15s
build-macos (x86_64-apple-darwin, dev)	12m 2s
build-linux-stable	0s
build-linux-pr (release, armv7-unknown-linux-gnueabihf)	1m 57s
build-windows-master	0s
build-macos (x86_64-apple-darwin, release)	3m 58s
build-linux-pr (release, aarch64-linux-android)	2m 15s
build-macos (aarch64-apple-darwin, dev)	1m 24s
build-macos (aarch64-apple-darwin, release)	3m 28s
build-windows-stable	0s
build-windows-pr (x86_64-pc-windows-msvc, dev)	10m 24s
check	36s
build-windows-pr (x86_64-pc-windows-msvc, release)	4m 48s
doc	3s
conclusion	1s
	1h 29m 30s

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