improved error reporting when a missing online dependency can't be downloaded

ldm/models/diffusion/ddpm.py

@@ -17,6 +17,7 @@ from functools import partial
 from tqdm import tqdm
 from torchvision.utils import make_grid
 from pytorch_lightning.utilities.distributed import rank_zero_only
+import urllib
 
 from ldm.util import log_txt_as_img, exists, default, ismap, isimage, mean_flat, count_params, instantiate_from_config
 from ldm.modules.ema import LitEma
@@ -524,7 +525,10 @@ class LatentDiffusion(DDPM):
         else:
             assert config != '__is_first_stage__'
             assert config != '__is_unconditional__'
-            model = instantiate_from_config(config)
+            try:
+                model = instantiate_from_config(config)
+            except urllib.error.URLError:
+                raise SystemExit("* Couldn't load a dependency. Try running scripts/preload_models.py from an internet-conected machine.")
             self.cond_stage_model = model
 
     def _get_denoise_row_from_list(self, samples, desc='', force_no_decoder_quantization=False):