The watch --wait command could also wait if stack does not exist (yet)

[martinth]

A scenario I encounter from time to time:

• You push a change to a (fresh) feature branch
• CI takes a moment to start deploying the application stack for my branch
• I already type rain watch --wait my-branch

In that case rain reports

error watching stack 'my-branch': Stack with id my-branch does not exist

I think this could be changed to instead printing a warning (to help people that mistype the stack name) and then wait for the stack to appear. Since the watch command is purely intended for interactive use I think this would be a nice quality of life improvement.