Improve dbt Docs Hosting Debugging -- Update dbt_docs_not_set_up.html (…

…#1250)

## 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.

cosmos/plugin/templates/dbt_docs_not_set_up.html

@@ -1,6 +1,6 @@
 {% extends base_template %}
 {% block content %}
- <h1 style="display: block; padding-top: 15px; padding-bottom: 15px">⚠️ Your dbt docs are not set up yet! ⚠️</h1>
+ <h1 style="display: block; padding-top: 15px; padding-bottom: 15px">⚠️ Your dbt docs are not set up yet! dbt_docs_dir was not found!⚠️</h1>
 
  <p>
  Read the <a href="https://astronomer.github.io/astronomer-cosmos/configuration/hosting-docs.html">Astronomer Cosmos docs</a> for information on how to set up dbt docs.