FileStorage: Report problem on read-only open of non-existent file
... 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.
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