Errors at the top of an expression are printed normally:

  nix-repl> builtins.throw "Evil puppy detected!!!"
  error:
         … while calling the 'throw' builtin
           at «string»:1:1:
              1| builtins.throw "Evil puppy detected!!!"
               | ^

         error: Evil puppy detected!!!

Errors in attribute values are printed inline, to make it easier to explore
values like nixpkgs where some parts of the value fail to evaluate:

  nix-repl> { puppy = builtins.throw "This puppy is EVIL!!!"; puppy2 = "This puppy is GOOD :)"; }
  {
    puppy = «error: This puppy is EVIL!!!»;
    puppy2 = "This puppy is GOOD :)";
  }

Same for list values:

  nix-repl> [ (builtins.throw "This puppy is EVIL!!!") ("This puppy is GOOD :)") ]
  [
    «error: This puppy is EVIL!!!»
    "This puppy is GOOD :)"
  ]