Raise initial HTTPError if pipeline is not cached locally (#4230)

* Raise initial HTTPError if pipeline is not cached locally

* make style

src/diffusers/pipelines/pipeline_utils.py

@@ -1248,6 +1248,7 @@ class DiffusionPipeline(ConfigMixin):
         allow_patterns = None
         ignore_patterns = None
 
+        model_info_call_error: Optional[Exception] = None
         if not local_files_only:
             try:
                 info = model_info(
@@ -1258,6 +1259,7 @@ class DiffusionPipeline(ConfigMixin):
             except HTTPError as e:
                 logger.warn(f"Couldn't connect to the Hub: {e}.\nWill try to load from local cache.")
                 local_files_only = True
+                model_info_call_error = e  # save error to reraise it if model is not cached locally
 
         if not local_files_only:
             config_file = hf_hub_download(
@@ -1389,7 +1391,8 @@ class DiffusionPipeline(ConfigMixin):
             user_agent["custom_pipeline"] = custom_pipeline
 
         # download all allow_patterns - ignore_patterns
-        cached_folder = snapshot_download(
+        try:
+            return snapshot_download(
                 pretrained_model_name,
                 cache_dir=cache_dir,
                 resume_download=resume_download,
@@ -1401,8 +1404,21 @@ class DiffusionPipeline(ConfigMixin):
                 ignore_patterns=ignore_patterns,
                 user_agent=user_agent,
             )
-
-        return cached_folder
+        except FileNotFoundError:
+            # Means we tried to load pipeline with `local_files_only=True` but the files have not been found in local cache.
+            # This can happen in two cases:
+            # 1. If the user passed `local_files_only=True`                    => we raise the error directly
+            # 2. If we forced `local_files_only=True` when `model_info` failed => we raise the initial error
+            if model_info_call_error is None:
+                # 1. user passed `local_files_only=True`
+                raise
+            else:
+                # 2. we forced `local_files_only=True` when `model_info` failed
+                raise EnvironmentError(
+                    f"Cannot load model {pretrained_model_name}: model is not cached locally and an error occured"
+                    " while trying to fetch metadata from the Hub. Please check out the root cause in the stacktrace"
+                    " above."
+                ) from model_info_call_error
 
     @staticmethod
     def _get_signature_keys(obj):