local-derivation-goal: improve "illegal reference" error

Before the change "illegal reference" was hard to interpret as it did
not mention what derivation actually hits it.

Today's `nixpkgs` example:

Before the change:

    $ nix build --no-link -f. postgresql_14
    ...
    error: derivation contains an illegal reference specifier 'man'

After the change:

    $ nix build --no-link -f. postgresql_14
    ...
    error: derivation '/nix/store/bxp6g57limvwiga61vdlyvhy7i8rp6wd-postgresql-14.15.drv' output 'lib' contains an illegal reference specifier 'man', expected store path or output name (one of [debug, dev, doc, lib, out])

src/libstore/unix/build/local-derivation-goal.cc

@@ -2927,8 +2927,17 @@ void LocalDerivationGoal::checkOutputs(const std::map<std::string, ValidPathInfo
                         spec.insert(worker.store.parseStorePath(i));
                     else if (auto output = get(outputs, i))
                         spec.insert(output->path);
-                    else
-                        throw BuildError("derivation contains an illegal reference specifier '%s'", i);
+                    else {
+                        std::string allOutputs;
+                        for (auto & o : outputs) {
+                            if (! allOutputs.empty())
+                                allOutputs.append(", ");
+                            allOutputs.append(o.first);
+                        }
+                        throw BuildError("derivation '%s' output '%s' contains an illegal reference specifier '%s',"
+                            " expected store path or output name (one of [%s])",
+                            worker.store.printStorePath(drvPath), outputName, i, allOutputs);
+                    }
                 }
 
                 auto used = recursive