Improve nix-store --delete failure message

On several occasions I've found myself confused when trying to delete
a store path, because I am told it's still alive, but
nix-store --query --roots doesn't show anything.  Let's save future
users this confusion by mentioning that a path might be alive due to
having referrers, not just roots.

src/libstore/gc.cc

@@ -781,7 +781,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
                 throw Error(
                     "Cannot delete path '%1%' since it is still alive. "
                     "To find out why, use: "
-                    "nix-store --query --roots",
+                    "nix-store --query --roots and nix-store --query --referrers",
                     printStorePath(i));
         }