The mypy job complaints about 'exc' variable[1] since it was used
for ExceptionChainer as well as TargetPortalNotFound exception.
Changing the variable name for TargetPortalNotFound exception from
'exc' to 'target_exc' makes the 'type: ignore' comments unnecessary.
[1] Trying to read deleted variable 'exc'
Change-Id: I4b10db0754f0e00bb02d3a60f9aaf88b90466a8f