multi: rename FailUnknownPaymentHash to FailIncorrectDetails

Align naming better with the lightning spec. Not the full name of the
failure (FailIncorrectOrUnknownPaymentDetails) is used, because this
would cause too many long lines in the code.

lntest/itest/lnd_test.go

@@ -8664,7 +8664,7 @@ out:
 		t.Fatalf("payment should have been rejected due to invalid " +
 			"payment hash")
 	}
-	expectedErrorCode := lnwire.CodeUnknownPaymentHash.String()
+	expectedErrorCode := lnwire.CodeIncorrectOrUnknownPaymentDetails.String()
 	if !strings.Contains(resp.PaymentError, expectedErrorCode) {
 		// TODO(roasbeef): make into proper gRPC error code
 		t.Fatalf("payment should have failed due to unknown payment hash, "+
@@ -8696,7 +8696,7 @@ out:
 		t.Fatalf("payment should have been rejected due to wrong " +
 			"HTLC amount")
 	}
-	expectedErrorCode = lnwire.CodeUnknownPaymentHash.String()
+	expectedErrorCode = lnwire.CodeIncorrectOrUnknownPaymentDetails.String()
 	if !strings.Contains(resp.PaymentError, expectedErrorCode) {
 		t.Fatalf("payment should have failed due to wrong amount, "+
 			"instead failed due to: %v", resp.PaymentError)