htlcswitch: if we detect an InvalidCommitSigError, send over detailed error
htlcswitch: if we detect an InvalidCommitSigError, send over detailed error

In this commit, we add an additional case when handling a failed
commitment signature. If we detect that it’s a InvalidCommitSigError,
then we’ll send over an lnwire.Error message with the full details. We
don’t yet properly dispatch this error on the reciting side, but that
will be done in a follow up a commit.

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