FCPClientReferenceImplementation: Fix handling
of SubscriptionExistsAlready FCP error

- Instead of only logging the receival of the error, mark the
subscription as succeeded so we don't try to file it again. (=Updating
our member variables as the removed FIXME demands)
- Also replace the old comment which explains when this situation can
happen with a much better explanation. The old explanation was mostly
wrong anyway IMHO.
- I've done a test run, and the parsing of the unmodified message by the
new FCPSubscriptionSuccedeedHandler() works.