configuration: improved error message when config file is missing

chatmastermind/configuration.py

@@ -150,6 +150,8 @@ class Config:
 
     @classmethod
     def from_file(cls: Type[ConfigInst], path: str) -> ConfigInst:
+        if not Path(path).exists():
+            raise ConfigError(f"Configuration file '{path}' not found. Use 'cmm config --create' to create one.")
         with open(path, 'r') as f:
             source = yaml.load(f, Loader=yaml.FullLoader)
         return cls.from_dict(source)