bubble up error for missing .readthedocs.yml if explicitly configured

Author: ewdurbin

readthedocs/config/config.py

@@ -1380,7 +1380,9 @@ def load(path, env_config, config_file=None):
     if config_file is None or config_file == '':
         filename = find_one(path, CONFIG_FILENAME_REGEX)
     else:
-        filename = "/".join([path, config_file])
+        filename = os.path.join(path, config_file)
+        if not os.path.exists(filename):
+            raise ConfigError(f".readthedocs.yml not found at {config_file}", CONFIG_FILE_REQUIRED)
 
     if not filename:
         raise ConfigFileNotFound(path)