Add `FailureCode::InvalidOnionPayload` variant

Add `FailureCode::InvalidOnionPayload` variant

When a user decodes custom TLVs, if they fail to recognize even type
numbers they should fail back with the correct failure code and fail
data. This new variant adds the proper failure variant for the user to
pass into `ChannelManager::fail_htlc_backwards_with_reason`.

Note that the enum discriminants were removed because when adding a
struct variant we can no longer make use of the discriminant through
casting like we previously did, and instead have to manually define the
associated failure code anyway.