Make secrets.json not being found an explicit error when DJANGOPROJECT_DATA_DIR is set

[alexgmin]

While helping someone setup the website in their system, they faced an issue in which the secrets.json file wasn't found, and they had DJANGOPROJECT_DATA_DIR env var set (but the secrets.json was on a different directory).

The instructions tell you to have this on your bashrc/zshrc export DJANGOPROJECT_DATA_DIR=~/.djangoproject, and if set, expect a ~/.djangoproject/conf/secrets.json.
In fact there's a fallback for that but only in the env variable is not set, which the instructions never tell you about.

Removing the DJANGOPROJECT_DATA_DIR would require coordination with operations to make sure it doesn't break their setup, so for now I'd just make it error explicitly if the secrets.json file is not found, which is a good initial step.

I think there's a case in which this breaks an existing working local setup, which requires the following conditions:

1. The env var DJANGOPROJECT_DATA_DIR is set to a path that doesn't contain secrets.json. Note that the website can work without DJANGOPROJECT_DATA_DIR being set at all.
2. The local postgres is set up to use exactly the defaults set in DATABASES. Any difference will mean it will raise an error when trying to execute Django.

[adamzap]

👍 Looks great!