If a slug is already taken, show a better error, and prompt for a new slug

We don’t handle slug conflicts gracefully on deploy:

```
◇  What slug do you want to use?
│  hello-framework
Could not create project: Unexpected response status 409
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
```

Can we do async validation of slugs? I think we can.

Related #482.