Exit process with error if there are b1 or bz1 shards

[pauldix]

The 0.11.0 release removed support for b1 and bz1 shards entirely. This means that users upgrading will have to convert their old shards before starting up 0.11.0.

To ensure that upgrading users don't see wonky behavior, we should check for the existence of old b1 and bz1 shards on startup. If some are found, we should exit the process with an error message pointing them to the upgrade info.