Improve various error messages #6878

Job	Run time
build-compiler (ubuntu-24.04, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, linux...	9m 11s
build-compiler (ubuntu-24.04-arm, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, t...	9m 5s
build-compiler (macos-13, 5.3.0, true, darwin-x64, x86_64-apple-darwin)	9m 20s
build-compiler (macos-14, 5.3.0, true, darwin-arm64, aarch64-apple-darwin)	9m 3s
build-compiler (windows-latest, 5.3.0, true, win32-x64, x86_64-pc-windows-gnu)	8m 59s
build-compiler (ubuntu-24.04, ocaml-variants.5.2.1+options,ocaml-option-static, linux-x64, x86_64...	9m 12s
build-compiler (ubuntu-24.04, ocaml-variants.5.0.0+options,ocaml-option-static, linux-x64, x86_64...	9m 12s
build-compiler (ubuntu-24.04, ocaml-variants.4.14.2+options,ocaml-option-static, linux-x64, x86_6...	9m 11s
pkg-pr-new	1s
test-integration	1s
publish	1s
	1h 13m 16s

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