Fixes https://github.com/rails/rails/issues/42196

Now, `rails new` will display the same error message that `rails` and `rails help new` does.

To test:

- Pull this branch
- Uninstall your local rails copy using `gem uninstall rails`
- Move into the root directory where you pulled down the rails code, then install: `rake install`
- In another directory run `rails new`