Fix race when preparing download location

If multiple threads start their downloads into the same directory, they can
call os.mkdirs at the same time, leading to "File exists" exceptions and
subsequent failures to download some files. Fix this by supplying an exist_ok
argument and removing the racy isdir check.

minigalaxy/download_manager.py

@@ -317,8 +317,7 @@ def __prepare_location(self, save_location):
         """
         # Make sure the directory exists
         save_directory = os.path.dirname(save_location)
-        if not os.path.isdir(save_directory):
-            os.makedirs(save_directory, mode=0o755)
+        os.makedirs(save_directory, mode=0o755, exist_ok=True)
 
         # Fail if the file already exists
         if os.path.isdir(save_location):