Translate all not found errors to ipld ErrNotFound for bitswap

[dirkmc]

We are seeing a lot of extraneous "not found" errors in the logs:

2022-10-04T16:13:59.899+0300    ERROR   engine  decision/blockstoremanager.go:99        blockstore.GetSize(QmegVFK7NszHq6BG3Lu2kKQ6YkKjyhWwaZ28i2M2mCRPQi) error: block not found
2022-10-04T16:13:59.899+0300    ERROR   engine  decision/blockstoremanager.go:99        blockstore.GetSize(bafkreiccq5bf6vm7pmnvz26hcqmkbtpayg3srhpsenq6dzy2565qjbt6wy) error: block not found
2022-10-04T16:13:59.899+0300    ERROR   engine  decision/blockstoremanager.go:99        blockstore.GetSize(bafybeicoprmqmoztnidjg4u5gqg33cz36heglyoyl6lgeznsvg5z7pdwty) error: block not found

This is because

• bitswap expects ipld ErrNotFound if the block is not found
• different parts of the system return different "not found" errors (eg ipld ErrNotFound, blockstore ErrNotFound)
• the error type is not preserved across the RPC boundary

This PR just checks if the error message contains the string "not found", and if so, returns it to bitswap as an ipld ErrNotFound