improve errors for missing directories (#1765)

* improve errors for missing directories

Fixes #1761
Updates #1760

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

* update container docs

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

* update changelog with /var changes

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

---------

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/util/file.go

@@ -1,6 +1,8 @@
 package util
 
 import (
+	"errors"
+	"fmt"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -42,3 +44,21 @@ func GetFileMode(key string) fs.FileMode {
 
 	return fs.FileMode(mode)
 }
+
+func EnsureDir(dir string) error {
+	if _, err := os.Stat(dir); os.IsNotExist(err) {
+		err := os.MkdirAll(dir, PermissionFallback)
+		if err != nil {
+			if errors.Is(err, os.ErrPermission) {
+				return fmt.Errorf(
+					"creating directory %s, failed with permission error, is it located somewhere Headscale can write?",
+					dir,
+				)
+			}
+
+			return fmt.Errorf("creating directory %s: %w", dir, err)
+		}
+	}
+
+	return nil
+}