Make `switch` command a bit more helpful

The `switch` command currently says:

~~~
14:29:39 $ gl switch feature/foo
✘ Branch feature/foo doesn't exist
  ➜ to list existing branches do gl branch
~~~

It should say:

~~~
14:29:39 $ gl switch feature/foo
✘ Branch feature/foo doesn't exist
  ➜ to list existing branches do gl branch
  ➜ to create a new branch do gl branch -c feature/foo
~~~
