dtlib: type annotate _err()

Marking this NoReturn helps the type checker figure out that functions
which call it are only returning valid values or failing to
return. (It unfortunately doesn't always work as mypy's control flow
analysis seems to treat a direct 'raise DTError(...)' differently than
calling _err() in some situations, but it helps.)

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>