fix: improve error handling for tracing policies directory access

[arthur-zhang]

improve error display when policy dir do not exist when starting tetragon.

Description

Before the change，the error message is not clear to figure out what happens when custom tracing policy dir do not exist, the error message is

time="2025-01-10T10:28:37+08:00" level=fatal msg="Failed to start tetragon" error="stat .: no such file or directory"

Now we check all directories upfront and provide better error context.

time="2025-01-10T10:38:06+08:00" level=fatal msg="Failed to start tetragon" error="Tracing Policies directory does not exist: /home/arthur/dev/tetragon/my_policy_dir"

Changelog

Optimize directory existence check in loadTpFromDir

[mtardy]

Hey thanks! Not related to your change but the existing code: I'm not a huge fan of this flow as it looks unnecessarily complicated and I don't exactly get why we don't want to fail in a situation where the directory is given but does not exists. @tixxdz do you remember why you added this "return nil" case? Should we keep this?

You could simplify this a bit as suggested, it's a less explicit error msg but the code flow is a bit more understandable.

[mtardy]

Suggested change

	if os.IsNotExist(err) {
	// If the default directory does not exist then do not fail
	// Probably tetragon not fully installed, developers testing, etc
	if dir == defaults.DefaultTpDir {
	log.WithField("tracing-policy-dir", dir).Info("Loading Tracing Policies from directory ignored, directory does not exist")
	return nil
	}
	return fmt.Errorf("Tracing Policies directory does not exist: %s", dir)
	}
	return fmt.Errorf("Failed to access tracing policies directory: %w", err)
	// Do not fail if the default directory doesn't exist,
	// it might because of developer setup or incomplete installation
	if os.IsNotExist(err) && dir == defaults.DefaultTpDir {
	log.WithField("tracing-policy-dir", dir).Info("Loading Tracing Policies from directory ignored, directory does not exist")
	return nil
	}
	return fmt.Errorf("Failed to access tracing policies dir %s: %w", dir, err)