This is a follow up for https://review.opendev.org/#/c/738145/
During backporting review, it became clear that unit test had a
flaw. It assumed that order of items in dictionary that make up
the exception message did not change. That is not true, based
on the python version used.
This follow up also includes a review feedback that did not make
into the original change: rename function that raises exception
to have "raise" in its name (raise_port_forwarding_update_failed).
Change-Id: I6fcd64e205e584017e6c9022f82a5497ea1cc576
Closes-Bug: #1878299
838399f0a4