Add error handling to config_schema.py

[afwillia]

Is your feature request related to a problem? Please describe.
If deploying to shinyapps.io with a data model without any DependsOn Component attributes, the authorize and deploy step will fail with missing package errors of the form

* Application depends on package "DBI" but it is not installed. Please
  resolve before continuing.

Fixing the data model resolves this error. What I've noticed is that config_schema.py will output a blank skeleton config.yml file with a bad data model. The script does not error out, it just creates an unusable file. It would be helpful to for the script to fail or warn when empty output is created.

How important is this feature? Select from the options below:
• 🌗 Medium - can do work without it; but it's important (e.g. to save time or for convenience)