[1] Updated the migration script to check for config-download
directory instead of stack, but missed update the Error
message.
check_stack function is renamed to check_source_inventory
as now it only checks for source inventory instead of heat
stack.
If source inventory file doesn't exist the script
will report Error message and exit.
This is follow up of [1].
[1] https://review.opendev.org/c/openstack/neutron/+/834925
Related-Bug: #1966099
Change-Id: I2416fba50fc495da4d59a3f335f33d831ca6e91d
(cherry picked from commit ccf8e71efa)
yatinkarel
435c869f1a