IndexUnreadable inherits FileNotFoundError #133

Found in PR #123 review round 3 (R3-6). IndexUnreadable(IndexMissing(FileNotFoundError)): a locked or permission-denied index is an OSError with errno None. Any except OSError/except FileNotFoundError around a select/connect call (reconcile.main already catches (OSError, ValueError) for registry loading) would report a lock as a missing file. Fix candidate: a non-OSError base class with IndexMissing and IndexUnreadable as siblings, caught together by the CLIs.