blockstore: Move BlockstoreError to separate file

[steviez]

Problem

The BlockstoreError type lives in blockstore_db.rs which is currently a bit of a junk drawer

Summary of Changes

Work towards better organization by moving BlockstoreError (and the related Result) to their own file

[mergify]

If this PR represents a change to the public RPC API:

1. Make sure it includes a complementary update to rpc-client/ (example)
2. Open a follow-up PR to update the JavaScript client @solana/web3.js (example)

Thank you for keeping the RPC clients in sync with the server API @steviez.

[steviez]

If this PR represents a change to the public RPC API:

No change to RPC API, just importing the type from the more "public" location