cnct: be stricter about matching preimages
cnct: be stricter about matching preimages

The former tryApplyPreimage function silently ignored invalid preimages.
This could mask potential bugs. This commit makes the logic stricter and
generates an error in case an unexpected mismatch occurs.

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