trace when the foo part of foo.bar.baz errors

Turns errors like:

let
  errpkg = throw "invalid foobar";
in errpkg.meta

error:
       … while calling the 'throw' builtin
         at «string»:2:12:
            1| let
            2|   errpkg = throw "invalid foobar";
             |            ^
            3| in errpkg.meta

       error: invalid foobar

into errors like:

let
  errpkg = throw "invalid foobar";
in errpkg.meta

error:
       … while evaluating 'errpkg' to select 'meta' on it
         at «string»:3:4:
            2|   errpkg = throw "invalid foobar";
            3| in errpkg.meta
             |    ^

       … while calling the 'throw' builtin
         at «string»:2:12:
            1| let
            2|   errpkg = throw "invalid foobar";
             |            ^
            3| in errpkg.meta

       error: invalid foobar

For the low price of one try/catch, you too can have the incorrect line
of code actually show up in the trace!

Change-Id: If8d6200ec1567706669d405c34adcd7e2d2cd29d

src/libexpr/eval.cc

@@ -1432,7 +1432,20 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     Value * vCurrent = &vFirst;
     // Position for the current attrset Value in this select chain.
     PosIdx posCurrent;
-    e->eval(state, env, vFirst);
+
+    try {
+        e->eval(state, env, vFirst);
+    } catch (Error & e) {
+        assert(this->e != nullptr);
+        state.addErrorTrace(
+            e,
+            getPos(),
+            "while evaluating '%s' to select '%s' on it",
+            ExprPrinter(state, *this->e),
+            showAttrPath(state.symbols, this->attrPath)
+        );
+        throw;
+    }
 
     try {
         auto dts = state.debugRepl