More useful errors when sending fails

[pm47]

Our sending code results in generic NoAvailableChannels errors when a failure is returned by the channel (Commitment.sendAdd()). This logic seems to be inherited from eclair and makes sense in a multi-channels context, but not in lightning-kmp where we basically have 0 or 1 channel in Normal state. It would be much more helpful to return the error from the last channel tried.