htlcswitch/link: return InvalidOnionPayload failure
htlcswitch/link: return InvalidOnionPayload failure

This commit modifies the link return an InvalidOnionPayload failure when
it cannot parse a TLV payload. The offset is left at zero, since its
unclear how useful it will be in practice and would require some
significant reworkings of the abstractions in the tlv package.

TODO: add unit tests. currently none of the test unit infrastructure is
setup to handle TLV payloads, so this would require implementing a
separate mock iterator for TLV payloads that also supports injecting
invalid payloads. Deferring this non-trival effor till a later date

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