... instead of silently creating empty database on such opens.

Use-case for this are utilities like e.g. zodbdump and zodbcmp which
expect such storage opens to fail so that the tool can know there is no
such storage and report it to user.

In contrast current state is: read-only opens get created-on-the-fly
empty storage with no content, but which can be iterated over without
getting any error.

This way e.g. `zodbdump non-existent.fs` produces empty output _and_
exit code 0 which is not what caller expects.

(cherry picked from commit 30bbabf1)