fix: add default PIL font as fallback (ultralytics#7010)

* fix: add default font as fallback

Add default font as fallback if the downloading of the Arial.ttf font
fails for some reason, e.g. no access to public internet.

* Update plots.py

Co-authored-by: Maximilian Strobel <Maximilian.Strobel@infineon.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

utils/plots.py

@@ -7,6 +7,7 @@
 import os
 from copy import copy
 from pathlib import Path
+from urllib.error import URLError
 
 import cv2
 import matplotlib
@@ -55,11 +56,13 @@ def check_pil_font(font=FONT, size=10):
     try:
         return ImageFont.truetype(str(font) if font.exists() else font.name, size)
     except Exception:  # download if missing
-        check_font(font)
         try:
+            check_font(font)
             return ImageFont.truetype(str(font), size)
         except TypeError:
             check_requirements('Pillow>=8.4.0')  # known issue https://github.com/ultralytics/yolov5/issues/5374
+        except URLError:  # not online
+            return ImageFont.load_default()
 
 
 class Annotator: