Show import error for 'airflow dags list' CLI command

[ephraimbuddy]

When there's an import error in a dag, the dag doesn't show up in the list of dags
and there's no indication that there's an import error.
This PR fixes that

[mik-laj]

Won't that cause problem for scripts that process the output of this command? I think we should display import errors on stderr.

[mik-laj]

Related PR: #15087

[ephraimbuddy]

Related PR: #15087

What's your opinion about having --show-import-error option? I think printing the error in stderr without any option is OK.
Happy to hear what you think then I would work on it and add tests

[mik-laj]

@ephraimbuddy, For me, displaying an error on stderr without any options is also OK. In the above-mentioned PR, I proposed to add a separate command as this would allow us to write scripts that process these import errors, but we can do it as a separate contribution.

[ephraimbuddy]

@mik-laj I have created a separate command for import errors(PR: #22084). Would wait for that to be merged and update this PR to just say that we have import errors and ask the users to run airflow dags list-import-errors to see the errors. WDYT?

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.