The scheduler command dagster schedule up should warn when DAGSTER_HOME is missing

[helloworld]

When you run the command dagster schedule up but don't have the environment variable DAGSTER_HOME set, you get the following error message:

dagster schedule up
Usage: dagster schedule up [OPTIONS]

Error: A scheduler must be configured to run schedule commands.
You can configure a scheduler on your instance using dagster.yaml.
For more information, see:

https://docs.dagster.io/deploying/instance/#scheduler

This error message should only really be displayed if you have DAGSTER_HOME set, but still haven't configured a scheduler. We might want to add a small message letting the user know about DAGSTER_HOME on top of the current information about how to set up the scheduler in dagster.yaml

From the discussion: #2946