use bb8 error sink to log more errors

[davepacheco]

I found while debugging #3207 that bb8 provides a way to receive database connection errors that aren't associated with a checked-out connection. We should use this to report those errors.

[davepacheco]

I tested this by running omicron-dev db-run, then starting Nexus, then killing the db-run command. Nexus emitted eight lines of this:

03:00:37.734Z ERRO e6bff1ff-24fb-49dc-a54e-c6a350cd4d6c (ServerContext): database connection error
    database_url = postgresql://[email protected]:32221/omicron?sslmode=disable
    error_message = Failed to checkout a connection: server is shutting down

The error message is a little misleading because we weren't trying to checkout a connection. I will take a look at fixing this up in async-bb8-diesel. But that's unrelated to this change.

[sunshowers]

Awesome, thanks!