FTP callable `download_all()` fails when logging to Slack

[jayckaiser]

When the FTP callable download_all() fails and enters the exception clause, the following error is raised:

File "/home/airflow/code/ea_airflow_util/ea_airflow_util/callables/ftp.py", line 54, in download_all
    if slack_conn_id := context["dag"].user_defined_macros.get("slack_conn_id"):
                        ~~~~~~~^^^^^^^
KeyError: 'dag'

This code was originally copied from a Rally implementation and likely has not been updated in some time. We should replace this error-logging to Slack with one of the built-in Slack alert callables.