htlcswitch: rename FailureDetail to OutgoingFailure

Rename FailureDetail in a separate commit so that a FailureDetail
interface can be introduced in the following commit.
OutgoingFailureOnionDecode is renamed to OutgoingFailureDecodeError
to specifically indicate that we could not decode the wire
failure that our payment experienced.

htlcswitch/link.go

@@ -2284,7 +2284,7 @@ func (l *channelLink) canSendHtlc(policy ForwardingPolicy,
 				return lnwire.NewTemporaryChannelFailure(upd)
 			},
 		)
-		return NewDetailedLinkError(failure, FailureDetailHTLCExceedsMax)
+		return NewDetailedLinkError(failure, OutgoingFailureHTLCExceedsMax)
 	}
 
 	// We want to avoid offering an HTLC which will expire in the near
@@ -2319,7 +2319,7 @@ func (l *channelLink) canSendHtlc(policy ForwardingPolicy,
 			},
 		)
 		return NewDetailedLinkError(
-			failure, FailureDetailInsufficientBalance,
+			failure, OutgoingFailureInsufficientBalance,
 		)
 	}