nix/loader: improve error messages #30

Job	Run time
lint-build	1m 8s
test-flake (x86_64-unknown-linux-gnu)	1m 16s
rustfmt	7s
test-flake (x86_64-unknown-linux-musl)	1m 5s
test (stable, x86_64-unknown-linux-gnu)	2m 56s
test-wasm	1m 3s
test (stable, x86_64-unknown-linux-musl)	5m 29s
test-flake (aarch64-unknown-linux-musl)	57s
test-flake (x86_64-apple-darwin)	4m 4s
test (stable, aarch64-unknown-linux-musl)	2m 3s
test (stable, x86_64-apple-darwin)	6m 6s
test-flake (aarch64-apple-darwin, macos-14)	1m 46s
test (stable, x86_64-pc-windows-msvc)	6m 9s
test (stable, aarch64-apple-darwin, macos-14, false, true, true)	3m 31s
test-projects	1m 35s
	39m 15s

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