Allow custom error messages to be passed to authorize call

- Add Readme section on rescuing from `Pundit::NotAuthorizedError` in Rails.
- Allow custom error messages to be set in controller actions.

Regarding the custom error messages, I think that passing in a custom error message in the controller action should suffice.

```ruby
authorize @post, :create?, "You cannot create new posts."
```

 #20 and #32 proposed custom `error_message(query)` methods on the policy, but I'm unsure whether this will bring us any further. Would you check the implementation and docs section @jnicklas and add in any other ways you might be rescuing a `Pundit::NotAuthorizedError`?

Closes #20 and #32.