Add $W back to the welcome prompt (both success and error)

[ekinanp]

This way if people cd to something outside of Wash root, they can easily get back to it.

[ekinanp]

Now that I think about it, I don't know if this will actually solve anything. We tell people to try docs, docs's output mentions W so it's there and is something people will see. Yet people still don't see it.

I think this is an issue of "I've done a bunch of stuff with Wash that I forgot what the prompt says, but I'd like an idiomatic way to cd back to Wash root". Latter seems to be done with cd