multi: deprecate IncorrectHtlcAmount onion error

In this commit, we deprecate the `IncorrectHtlcAmount` onion error.
We'll still decode this error to use when retrying paths, but we'll no
longer send this ourselves. The `UnknownPaymentHash` error has been
amended to also include the value of the payment as well. This allows us
to worry about one less error.

htlcswitch/link.go

@@ -2318,7 +2318,9 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 			if err != nil {
 				log.Errorf("unable to query invoice registry: "+
 					" %v", err)
-				failure := lnwire.FailUnknownPaymentHash{}
+				failure := lnwire.NewFailUnknownPaymentHash(
+					pd.Amount,
+				)
 				l.sendHTLCError(
 					pd.HtlcIndex, failure, obfuscator, pd.SourceRef,
 				)
@@ -2367,7 +2369,9 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 					"amount: expected %v, received %v",
 					invoice.Terms.Value, pd.Amount)
 
-				failure := lnwire.FailIncorrectPaymentAmount{}
+				failure := lnwire.NewFailUnknownPaymentHash(
+					pd.Amount,
+				)
 				l.sendHTLCError(
 					pd.HtlcIndex, failure, obfuscator, pd.SourceRef,
 				)
@@ -2394,7 +2398,9 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 					"got %v", pd.RHash, invoice.Terms.Value,
 					fwdInfo.AmountToForward)
 
-				failure := lnwire.FailIncorrectPaymentAmount{}
+				failure := lnwire.NewFailUnknownPaymentHash(
+					pd.Amount,
+				)
 				l.sendHTLCError(
 					pd.HtlcIndex, failure, obfuscator, pd.SourceRef,
 				)