That depends on what your error recovery plan is.

If the code's running in a space shuttle, you probably want to test that path.

If it's bootstrapping a replicated service, it's likely desirable to crash early if a config file couldn't be opened.

If it's plausible that the file in question is missing, you can absolutely test that code path, without mocking open.

If you want to explicitly handle different reasons for why opening a file failed differently, by all means, stress all of that in your tests. But if all you have is a happy path and an unhappy path, where your code doesn't care why opening a file failed, all you need to test is the case where the file is present, and one where it is not.