To support async signing, the InteractiveTxSigningSession returned when
handling tx_complete needs to be saved in order to retry the
funding_tx_constructed method when the signer is unblocked. This
unfortunately means an Option as needed since the unfunded V2 channel
phase variant is left unfunded until the signer completes.

When handling a tx_complete message, allow signers to return an error
indicating that the signer has not yet complete. This will leave the
ChannelPhase in an unfunded variant until the signer becomes unblocked.

These methods always return Ok, so there is no need to use a Result.

Expand ChannelManager::signer_unblocked to finish handling tx_complete
messages. This completes support for async signing in V2 channel
establishment.

The errs Vec in ChannelManager::claim_payment_internal hasn't been
populated since commit fea6393. Drop it
along with the code that consumed it.