routing: rename handleSendError to handleSwitchErr

This commit renames the `handleSendError` to be `handleSwitchErr` to
explicitly express that it's handling the error from htlcswitch.

routing/router.go

@@ -2365,7 +2365,7 @@ func (r *ChannelRouter) sendToRoute(htlcHash lntypes.Hash, rt *route.Route,
 	// the error to check if it maps into a terminal error code, if not use
 	// a generic NO_ROUTE error.
 	var failureReason *channeldb.FailureReason
-	err = sh.handleSendError(attempt, shardError)
+	err = sh.handleSwitchErr(attempt, shardError)
 
 	switch {
 	// If a non-terminal error is returned and `skipTempErr` is false, then