useful info for craft\db

[narration-sd]

Here's a description of what happens and what to do, from Slack posting. Would be nice if could be edited into coming doc...

a little tip for the unwary: if you're connecting to an external database such as mssql, don't use craft\db\Connection (or craft\db\Exception). You'll miss getting proper messages for a failed connection, even just server down.

I had the driver and connection fully working, save for server down, and thought it was a driver issue. But actually it's Craft's db code, and its handling if it doesn't recognize the database/driver type -- says simply unsupported connection type, and mentions the driver.

The real messages from the dblib driver are nicely enough detailed -- using yii\db\Connection (subclassed), and yii\db\Exception.