fix(errors): set env_name in error message correctly

src/repdav/errors.py

@@ -18,6 +18,6 @@ class EnvNotSetError(_ConfigError):
 
     def __init__(self, env_name):
         self.env_name = env_name
-        self.message = "Environment variable '%s' is not set."
+        self.message = f"Environment variable {env_name} is not set."
         _logger.error(self.message, self.env_name)
         super().__init__(self.message)