libexpr: include the type of the non-derivation value in the type error

Change-Id: Id4717b5b0df7c09b0dbf17e642d8713a0a3efbae

src/libexpr/get-drvs.cc

@@ -447,7 +447,8 @@ static void getDerivations(EvalState & state, Value & vIn,
         return;
     } else if (v.type() != nAttrs) {
         state.error<TypeError>(
-            "expression does not evaluate to a derivation (or a list or set of those)"
+            "expression was expected to be a derivation or collection of derivations, but instead was %s",
+            showType(v.type(), true)
         ).debugThrow();
     }