Improve dbt Docs Hosting Debugging -- Update dbt_docs_not_set_up.html

[johnmcochran]

Description

[ dbt_docs_dir is None ] directly triggers the [ dbt_docs_net_set_up.html ] page via the cosmos plugin. I had a hard time figuring out why my dbt docs weren't hosting properly considering I had (incorrectly) tried setting up the dbt_docs_dir variable via the Airflow UI. Once I added the environment variable to my Dockerfile, the dbt Docs showed correctly in the airflow UI. I had to inspect the source code for the plugin to find out that cosmos was not correctly seeing the environment variables I had set up.

Adding a note that the "dbt_docs_dir was not found" would make this error page much more helpful from a debugging standpoint for me.

Related Issue(s)

Related to #1245: Make it easier for end-users to create a DAG for creating/hosting dbt docs
Related to #1139: Write blog post dbt docs in Airflow with Cosmos

Breaking Change?

This is my first time creating a PR on a public github, but I think this is as low impact as you can get :)

Checklist

• [NA ] I have made corresponding changes to the documentation (if required)
• I have added tests that prove my fix is effective or that my feature works

I don't know how to test this :( Just trying to save others some time if they have the same problem I had with correctly hosting dbt docs.

[netlify]

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[tatiana]

Thank you for the detailed description and for improving this part of Cosmos, @johnmcochran ! It's a valuable contribution.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.73%. Comparing base (4c4c57c) to head (e1a2a34).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1250   +/-   ##
=======================================
  Coverage   95.73%   95.73%           
=======================================
  Files          67       67           
  Lines        3965     3965           
=======================================
  Hits         3796     3796           
  Misses        169      169           

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