This patch fixes an issue where errorMessage is not getting reassigned
from None in the method ._get_port_group_name_from_mv., having the knock
on effect where errors are not being picked up in subsequent calls or
conditional checks.
Change-Id: I74a85aaa912a7f4c22abbf48f0a5d1bd4538c098
Closes-Bug: #1686174
(cherry picked from commit b44f55c516)