update async-bb8-diesel for error message update

[davepacheco]

See oxidecomputer/async-bb8-diesel#43.

After this change and #3232, when I start up Nexus without Cockroach running, I see this log message:

[2023-05-25T20:18:38.178755567-07:00] ERROR: e6bff1ff-24fb-49dc-a54e-c6a350cd4d6c/ServerContext/2542 on ivanova: database connection error
    database_url: postgresql://[email protected]:32221/omicron?sslmode=disable
    --  
    error_message: Connection error: could not connect to server: Connection refused
        Is the server running on host "127.0.0.1" and accepting
        TCP/IP connections on port 32221?

When I have both Nexus and CockroachDB running and then shut down CockroachDB, I see this:

[2023-05-25T20:21:45.242147357-07:00] ERROR: e6bff1ff-24fb-49dc-a54e-c6a350cd4d6c/ServerContext/2610 on ivanova: database connection error (error_message="Connection error: server is shutting down")
    database_url: postgresql://[email protected]:32221/omicron?sslmode=disable

Before, these said something about failing to checkout a connection, which wasn't true.

Leaving as draft because I need to fix up Cargo.toml before landing it. But this is ready for review.