Add trace to build errors during import-from-derivation

Example:

error: builder for '/nix/store/9ysqfidhipyzfiy54mh77iqn29j6cpsb-failing.drv' failed with exit code 1;
       last 1 log lines:
       > FAIL
       For full logs, run 'nix log /nix/store/9ysqfidhipyzfiy54mh77iqn29j6cpsb-failing.drv'.

       … while importing '/nix/store/pfp4a4bjh642ylxyipncqs03z6kkgfvy-failing'

       at /nix/store/25wgzr2qrqqiqfbdb1chpiry221cjglc-source/flake.nix:58:15:

           57|
           58|         ifd = import self.hydraJobs.broken;
             |               ^
           59|

src/libexpr/primops.cc

@@ -118,6 +118,9 @@ static void import(EvalState & state, const Pos & pos, Value & vPath, Value * vS
             .msg = hintfmt("cannot import '%1%', since path '%2%' is not valid", path, e.path),
             .errPos = pos
         });
+    } catch (Error & e) {
+        e.addTrace(pos, "while importing '%s'", path);
+        throw e;
     }
 
     Path realPath = state.checkSourcePath(state.toRealPath(path, context));