dl_tarball.sh: Special handling of kernel-rt lacks error checking

[Scott Little]

Brief Description
-----------------
dl_tarball.sh contains special steps for handling kernel-rt frmo EPEL.
That code lacks error handling, and is hard coded to assume an EPEL 8
kernel. If a non-EPEL-8 kernel is supplied via an lst file, it will
appear to pass, but will likely have done something strange and unexpected.

Severity
--------
Major

Steps to Reproduce
------------------
insert a reference to an el7 or el9 kernel into a lst file and run
download_mirror.sh

Expected Behavior
------------------
kernel-rt*src.rpm is downloaded and contains the correct content.

Actual Behavior
----------------
kernel-rt*src.rpm is contains the incorrect content.

Reproducibility
---------------
100%

System Configuration
--------------------
N/A

Branch/Pull Time/Commit
-----------------------
March 4, 2022

Last Pass
---------
N/A.

Timestamp/Logs
--------------
N/A

Test Activity
-------------
Build

Workaround
----------
N/A

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/starlingx/tools/+/832676

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/starlingx/tools/+/832676
Committed: https://opendev.org/starlingx/tools/commit/feb596bdfdb4c8a5f921da648b3e219e779b7a93
Submitter: "Zuul (22348)"
Branch: master

commit feb596bdfdb4c8a5f921da648b3e219e779b7a93
Author: Scott Little <email address hidden>
Date: Tue Mar 8 12:37:17 2022 -0500

    dl_tarball.sh: safer downloads of kernel-rt

    dl_tarball.sh assumes an el8 kernel, and lacks of error
    handling. This may result in an attempted download of a
    non-el8 reporting success, but producing a src.rpm containing
    an el8 kernel instead.

    Testing:
    - Download current el8 rt kernel
    - Download previous el7 rt kernel
    - DL using invalid URL - error caught
    - DL using invalid sha - error caught

    Closes-Bug: 1964156
    Change-Id: I5ca5130a7f1c4e967c876060d95f9d6a2c2e8cf5
    Signed-off-by: Scott Little <email address hidden>