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