Use the "@" operator to silence connection retry messages if initializing the stack with database config optional

Summary:
Depends on D20780. Ref T13403. During initial setup, it's routine to run "bin/config" with a bad database config. We start the stack in "config optional" mode to anticipate this.

However, even in this mode, we may emit warnings if the connection fails in certain ways. These warnings aren't useful; suppress them with "@".

(Possibly this message should move from "phlog()" to "--trace" at some point, but it has a certain amount of context/history around it.)

Test Plan:

• Configured MySQL to fail with a retryable error, e.g. good host but bad port.
• Ran bin/config set ....
• Before: saw retry warnings on stderr.
• After: no retry warnings on stderr.
• (Turned off suppression code artificially and verified warnings still appear under normal startup.)

Maniphest Tasks: T13403

Differential Revision: https://secure.phabricator.com/D20781