When "mysqli->real_connect()" fails without setting an error code, recover more gracefully

Summary: Depends on D20779. Ref T13403. Bad parameters may cause this call to fail without setting an error code; if it does, catch the issue and go down the normal connection error pathway.

Test Plan:

• With "mysql.port" set to "quack", ran bin/storage probe.
• Before: wild mess of warnings as the code continued below and failed when trying to interact with the connection.
• After: clean connection failure with a useful error message.

Maniphest Tasks: T13403

Differential Revision: https://secure.phabricator.com/D20780