reduce nesting, better error reporting

gpt4all-bindings/python/gpt4all/gpt4all.py

@@ -87,7 +87,9 @@ class GPT4All():
         else:
             model_path = model_path.replace("\\", "\\\\")
 
-        if os.path.exists(model_path):
+        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)
@@ -102,8 +104,6 @@ class GPT4All():
             return GPT4All.download_model(model_filename, model_path)
         else:
             raise ValueError("Failed to retrieve model")
-        else:
-            raise ValueError("Invalid model directory")
 
     @staticmethod
     def download_model(model_filename: str, model_path: str) -> str: