Do not fail if FundTransaction was already mined

[luckysori]

After completing the channel creation protocol, both parties can broadcast the signed FundingTransaction. If both parties do so, depending on the timing, one might get an error from bitcoind saying that the transaction was already in the blockchain.

This currently fails the protocol as seen here, but it should not.

The same applies to the collaborative channel close protocol.

[bonomat]

That was to be expected :)