Load-import inactive does not return an assistive error.

[rafael moyano]

Title
-----
Load-import inactive does not return an assistive error.

Brief Description
-----------------
Importing an inactive load a second time returns an "Unknown Error" as opposed to an assistive error advising the user to first
remove the existing inactive load before importing a new one.

Severity
--------
Minor

Steps to Reproduce
------------------
1) Log in to CLI.
2) Source env file /etc/platform/openrc
3) Import a first load with inactive param set.
4) Verify load status with: system load-list.
5) Import a second load with inactive param set.

Expected Behavior
------------------
"Load import failed. Reason: An inactived load already exists. Please, remove the inactive load before trying to import a new one."

Actual Behavior
----------------
"Load import failed. Reason: Unknown Error"

Reproducibility
---------------
Reproducible 100%.

System Configuration
--------------------
DC.

Test Activity
-------------
System Test.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/starlingx/config/+/901171

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Rafael Moyano <email address hidden>" on branch: master
Review: https://review.opendev.org/c/starlingx/config/+/901171
Reason: This bug has been fixed by Lindley Vieira in this review: https://review.opendev.org/c/starlingx/config/+/899865