Display better error message when evaling an attr set

[SuperSandro2000]

Related #185

$ git -c fetch.prune=false fetch --no-tags --force https://github.com/NixOS/nixpkgs staging:refs/nixpkgs-review/0 pull/139808/head:refs/nixpkgs-review/1
From https://github.com/NixOS/nixpkgs
 + 9a4978f9cfb...40726812071 refs/pull/139808/head -> refs/nixpkgs-review/1  (forced update)
$ git worktree add /home/hotpi/.cache/nixpkgs-review/pr-139808-12/nixpkgs 845225e7ab34d00bae7b20f47d06c7418210a631
Preparing worktree (detached HEAD 845225e7ab3)
HEAD is now at 845225e7ab3 Merge pull request #147289 from midchildan/fix/libcxx13-darwin
$ git merge --no-commit --no-ff 40726812071e1fe0dbb9cb326b8bb4ab0cb4d7b6
Automatic merge went well; stopped before committing as requested
error: Tried building attr set with the following content: aarch64=/nix/store/bh24g8l4vlnf17hh6d6qp40i6fy3czvl-coreboot-toolchain-aarch64-4.15   arm=/nix/store/xaln448j4982kzcrqgfbxsd9wz888irb-coreboot-toolchain-arm-4.15   i386=/nix/store/3smj20p9kcsq7i0rcaqhgqijgr2p5ysk-coreboot-toolchain-i386-4.15   nds32le=/nix/store/ag1yry1rws27jw2dq1132818qfvm9wbm-coreboot-toolchain-nds32le-4.15   override=   overrideDerivation=   ppc64=/nix/store/l6wf06g65w768gkzqkcngr82nh5fjqqh-coreboot-toolchain-ppc64-4.15   recurseForDerivations=   riscv=/nix/store/b67nw64zv2dllcarb8cd6545akqgzwfn-coreboot-toolchain-riscv-4.15   x64=/nix/store/17y1agpgybp06f286m00gmdgc5vmcda4-coreboot-toolchain-x64-4.15
nix --experimental-features nix-command --system x86_64-linux eval --json --impure --expr (import /nix/store/zr8ajm22gqvb15qpwfq4z9g6izyw110z-nixpkgs-review/lib/python3.9/site-packages/nixpkgs_review/nix/evalAttrs.nix { allowAliases = false; attr-json = /tmp/tmpchah76g_; }) failed to run, /tmp/tmpchah76g_ was stored inspection
https://github.com/NixOS/nixpkgs/pull/139808 failed to build
$ git worktree prune

[Mic92]

Suggested change

	throw "Tried building attr set with the following content: ${debugAttr pkg}";
	throw "Cannot build attr set `${name}` with the following content:\n${debugAttr pkg}";

[Mic92]

What is the goal of function i.e. what cases are you trying to debug?

Could it be replaced by lib.generators.toPretty { allowPrettyValues = true; }?