shelve: raise an error when loading a corrupt state file in an impossible case The old return statement was flagged by pytype 2023.06.16 running under python 3.10.11. No idea why it isn't caught in CI running the same pytype with py3.7. This function is only called by `unshelvecmd()` (which first checks that either `--abort` or `--continue` is specified), and `hgabortunshelve()` and `hgcontinueunshelve()`, which locally apply `--abort` or `--continue` respectively. Therefore, there is no other way to call this, and this error should never be seen, but pytype can't figure that out on its own. Given that the abort case clears the state, it seems reasonable to defensively code this and not make that a blanket `else` case, on the off chance a 3rd way of calling this appears in the future.