Fold two stack trace messages in derivations

Combined with the previous changes, stack traces involving derivations
are now much less verbose, since something like

  while evaluating the builtin function `getAttr':
  while evaluating the builtin function `derivationStrict':
  while instantiating the derivation named `gtk+-2.24.20' at `/home/eelco/Dev/nixpkgs/pkgs/development/libraries/gtk+/2.x.nix:11:3':
  while evaluating the derivation attribute `propagatedNativeBuildInputs' at `/home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/default.nix:78:17':
  while evaluating the attribute `outPath' at `/nix/store/212ngf4ph63mp6p1np2bapkfikpakfv7-nix-1.6/share/nix/corepkgs/derivation.nix:18:9':
  ...

now reads

  while evaluating the attribute `propagatedNativeBuildInputs' of the derivation `gtk+-2.24.20' at `/home/eelco/Dev/nixpkgs/pkgs/development/libraries/gtk+/2.x.nix:11:3':
  ...

1 files changed, 2 insertions, 4 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 0585f5bbe..30dd15ac0 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -394,10 +394,8 @@ static void prim_derivationStrict(EvalState & state, Value * * args, Value & v)
             }
 
         } catch (Error & e) {
-            e.addPrefix(format("while evaluating the derivation attribute `%1%' at %2%:\n")
-                % key % *i->pos);
-            e.addPrefix(format("while instantiating the derivation named `%1%' at %2%:\n")
-                % drvName % posDrvName);
+            e.addPrefix(format("while evaluating the attribute `%1%' of the derivation `%2%' at %3%:\n")
+                % key % drvName % posDrvName);
             throw;
         }
     }