Implemented directory walk

errors/InvalidPath.go

@@ -0,0 +1,15 @@
+package errors
+
+import "fmt"
+
+type InvalidDirectory struct {
+	Path string
+}
+
+func (err *InvalidDirectory) Error() string {
+	if err.Path == "" {
+		return "Invalid directory"
+	}
+
+	return fmt.Sprintf("Invalid directory '%s'", err.Path)
+}