PHP-475: Functional tests for read preference exceptions

FYI: `mongocollection-find_error-001.phpt` has intermittent failures on my environment due to `replSetMaintenance` calls not sticking. Secondaries enter maintenance mode but drop out of it a second or two later, which leads to the driver sending them the query when we'd expect it not to. I get similar failures with the `mongod-recovery-mode-001.phpt` RS failover test.