Improve error message when using `start-airflow` with `--backend none` in Breeze

[kyungjunleeme]

Description

related https://apache-airflow.slack.com/archives/CQ9QHSFQX/p1750917889606339?thread_ts=1750915462.814829&cid=CQ9QHSFQX

Problem

Currently, when users run breeze --backend none and attempt to use start-airflow, it results in a cryptic error or unexpected behavior. This can be confusing, especially for newcomers who might not realize that the none backend is intended for non-DB tests and is not compatible with start-airflow.

Proposed Improvement

Improve the developer experience by adding a clearer error or warning message in start-airflow when the backend is set to none.

Use case/motivation

No response

Related issues

No response

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[kyungjunleeme]

I’d like to take this on if the maintainers are open to the proposed changes.

[kyungjunleeme]

@gopidesupavan Thank you