Automated tests for behaviour on I/O errors

From what I can see of the test suite, there do not seem to be any tests verifying that borg won't corrupt a repository if an unexpected I/O error of some kind occurs.

It would be really good to have some tests like this.

See [How SQLite Is Tested](https://www.sqlite.org/testing.html) for an impressive description of how they do it – this would be a great ultimate goal.

Meanwhile, it shouldn't be too hard to replace current explicit calls to `open()` etc. with a VFS layer that can be used to inject arbitrary I/O errors during testing.