Currently when you are trying to set invalid additional property for
task using py3 interpreter it will fail, because function `unicode` does
not exist in py3.
Fix it by replacing `unicode` with `utils.exception_to_str` which is
used in other modules already.
Change-Id: I5897868f801467a2eaa7585b5f2d578cef358426
Closes-Bug: 1439513