lnwallet/btcwallet: during initial creation catch the case of an existing wallet
lnwallet/btcwallet: during initial creation catch the case of an existing wallet

In this commit, due to the recent changes within lnd itself, it may be
possible that a wallet already exists when the wallet has been signaled
to be created. As a result, *always* open the wallet ourselves, but
allow an existing wallet to already be in place.

LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd