use digEmpty function in tunnel command

src/bin/TurtleCtl.hx

@@ -114,7 +114,7 @@ class TurtleCtl extends CLIAppBase {
 
 		registerAsyncSubcommand("tunnel", (args) -> {
 			var len = args.getInt("length");
-			return asynPerform(Helper.combine.bind([Turtle.dig.bind(Front), Turtle.forward, Turtle.dig.bind(Up)]), len);
+			return asynPerform(Helper.combine.bind([Turtle.digEmpty.bind(Front), Turtle.forward, Turtle.digEmpty.bind(Up)]), len);
 		}, [Int("length")]);
 	}
 

src/kernel/turtle/Turtle.hx

@@ -72,6 +72,11 @@ class Turtle {
 		return r2;
 	}
 
+	/**
+		Dig in the provided direction.
+		Keep in mind that digging on a empty space is considerd a failure.
+		Also see `digEmpty`.
+	**/
 	public static function dig(dir:InteractDirections, ?toolSide:ToolSide):Outcome<Noise, String> {
 		var r:cc.Turtle.TurtleActionResult;
 
@@ -87,6 +92,36 @@ class Turtle {
 		return conterToOutcome(r);
 	}
 
+	/**
+		Dig in the provided direction.
+		Does not fail if there is nothing to dig.
+		Also see `dig`.
+	**/
+	public static function digEmpty(dir:InteractDirections, ?toolSide:ToolSide):Outcome<Noise, String> {
+		var r:cc.Turtle.TurtleActionResult;
+
+		// FIXME: upstream needs to be fixed to accept ToolSide
+		switch dir {
+			case Front:
+				r = cc.Turtle.dig();
+			case Up:
+				r = cc.Turtle.digUp();
+			case Down:
+				r = cc.Turtle.digDown();
+		}
+		var result = conterToOutcome(r);
+
+		switch (result) {
+			case Success(_):
+				return result;
+			case Failure(failure):
+				if (failure == "Nothing to dig here") {
+					return Success(null);
+				}
+				return result;
+		}
+	}
+
 	public static function place(dir:InteractDirections):Outcome<Noise, String> {
 		var r:cc.Turtle.TurtleActionResult;
 		switch dir {