message: Make clear that the alias must exist when using `--alias`
diff: @@ -98,7 +98,7 @@ const help = () => {
  -e, --env Include an env var (e.g.: ${chalk.dim('`-e KEY=value`')}). Can appear many times.
  -C, --no-clipboard Do not attempt to copy URL to clipboard
  -N, --forward-npm Forward login information to install private NPM modules
  - -a, --alias Assign an alias to the deployment
  + -a, --alias Reassign an existing alias to the deployment
  ${chalk.dim('Enforcable Types (when both package.json and Dockerfile exist):')}
  