Otherwise, we would get non-standard txn's and fail to broadcast
them when cooperatively closing a channel. This wouldn't affect
funds security as no HTLCs would be active to steal. This is just
a safety measure as we should only generate standard txn's.
In order to be consistent with other sub systems an error is now
returned from the Stop functions.
This also allows writing a generic cleanup mechanism to stop all
sub systems in case of a failure.
This commit duplicates the utxonursery's writeOutpoint function
in the funding package so that when the rest of the fundingmanager
code is moved, it can use the WriteOutpoint function for its
channel opening state data.
