update-iso.sh has undefined log_error function

[Don Penney]

Brief Description
-----------------
The update-iso.sh utility references an undefined log_error function. As a result, error messages are not properly reported, instead resulting in messages such as:
stx-iso-utils.sh: line 53: log_error: command not found

Severity
--------
Minor

Steps to Reproduce
------------------
Run update-iso.sh with an error condition, such as missing required utilities.

Expected Behavior
------------------
Utility should report readable errors such as:

Unable to find required utility: rsync
Unable to find required utility: mkisofs
Unable to find required utility: isohybrid
Unable to find required utility: implantisomd5
One or more required utilities are missing. Aborting...

Actual Behavior
----------------
Utility reports:

stx-iso-utils.sh: line 53: log_error: command not found

Reproducibility
---------------
<Reproducible/Intermittent/Seen once>
State if the issue is 100% reproducible, intermittent or seen once. If it is intermittent, state the frequency of occurrence

System Configuration
--------------------
<One node system, Two node system, Multi-node system, Dedicated storage, https, IPv4, IPv6 etc.>

Branch/Pull Time/Commit
-----------------------
starlingx/master Sept 9, 2020

Last Pass
---------
Broken by the code reorg done in the following update:
https://review.opendev.org/707673

[Ghada Khalil (gkhalil)]

stx.5.0 / lower priority - would be nice to fix as this was unintentionally broken

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/750803
Committed: https://git.openstack.org/cgit/starlingx/utilities/commit/?id=6573fbe80c04e7cf85c46238c508aff1f2a04454
Submitter: Zuul
Branch: master

commit 6573fbe80c04e7cf85c46238c508aff1f2a04454
Author: Don Penney <email address hidden>
Date: Wed Sep 9 15:27:51 2020 -0400

    Fix missing log_error in update-iso.sh

    The update-iso.sh utility references an undefined log_error function.
    As a result, error messages are not properly reported, instead
    resulting in messages such as:
    stx-iso-utils.sh: line 53: log_error: command not found

    This error was introduced by code restructuring in a previous update:
    https://review.opendev.org/707673

    This update defines a log_error function for update-iso.sh that writes
    the message to stderr, as this is a user-run utility, so no need to
    log to syslog.

    Change-Id: Ifdc2a25c24cd86e61ccc12601a0bcbe6888ac349
    Closes-Bug: 1895042
    Signed-off-by: Don Penney <email address hidden>

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: f/centos8
Review: https://review.opendev.org/c/starlingx/utilities/+/792213