reduce nesting, better error reporting

gpt4all-bindings/python/gpt4all/gpt4all.py

@@ -87,24 +87,24 @@ class GPT4All():
         else:
             model_path = model_path.replace("\\", "\\\\")
 
-        if os.path.exists(model_path):
-            model_dest = os.path.join(model_path, model_filename).replace("\\", "\\\\")
-            if os.path.exists(model_dest):
-                print("Found model file at ", model_dest)
-                return model_dest
-
-            # If model file does not exist, download
-            elif allow_download: 
-                # Make sure valid model filename before attempting download
-                available_models = GPT4All.list_models()
-                if model_filename not in (m["filename"] for m in available_models):
-                    raise ValueError(f"Model filename not in model list: {model_filename}")
-                return GPT4All.download_model(model_filename, model_path)
-            else:
-                raise ValueError("Failed to retrieve model")
+        if not os.path.exists(model_path):
+            raise ValueError("Invalid model directory: {}".format(model_path))
+
+        model_dest = os.path.join(model_path, model_filename).replace("\\", "\\\\")
+        if os.path.exists(model_dest):
+            print("Found model file at ", model_dest)
+            return model_dest
+
+        # If model file does not exist, download
+        elif allow_download:
+            # Make sure valid model filename before attempting download
+            available_models = GPT4All.list_models()
+            if model_filename not in (m["filename"] for m in available_models):
+                raise ValueError(f"Model filename not in model list: {model_filename}")
+            return GPT4All.download_model(model_filename, model_path)
         else:
-            raise ValueError("Invalid model directory")
-        
+            raise ValueError("Failed to retrieve model")
+
     @staticmethod
     def download_model(model_filename: str, model_path: str) -> str:
         """