Fix test_reporawfilestore

This certainly gets the previously failing test to pass though it might be that the test is not the problem.

In the failing test, `testFailedWriteNoFile` calling a method on a closed rfs throws an exception but not an open one. In addition, the exception raised when trying to write to a read-only rfs is not the one originally expected.

In the other tests `rfs.close()` does not throw an NPE (as evidenced in other tests). Also a couple of `try` blocks used for exceptions have been converted to `with` blocks.

The only testing this PR needs is a clean pass in the integration tests. However, a review of what is expected by `testFailedWriteNoFile` by someone who know what the `RepoRawFileStore` is meant to do would be welcomed.

--no-rebase as this is develop-only