Add better help texts for Error::ShellProgramNotFound #8079

Job	Run time
rustfmt	4s
test (stable, x86_64-unknown-linux-gnu)	4m 7s
test-wasm	1m 20s
lint-build	47s
test (stable, x86_64-unknown-linux-musl)	5m 57s
validate	15s
test (stable, aarch64-unknown-linux-gnu)	1m 56s
test (stable, aarch64-unknown-linux-musl)	2m 49s
test (stable, x86_64-apple-darwin)	10m 35s
test (stable, x86_64-pc-windows-msvc)	6m 47s
test (stable, aarch64-apple-darwin, macos-latest, false, true)	3m 30s
test-projects	1m 54s
	40m 1s

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