Improve error message for connection errors

I recently had an issue connecting to a local project instance. It turned out to be my own fault, but in the process of debugging I wasn't able to get much information out of the cli as to exactly _what_ was going wrong. I only got the cryptic error message:

`edgedb error: Connection error: ClientConnectionFailedError`

It would be good to give a little more info here as to what is actually going wrong; is it a timeout? Incorrect DSN?

Perhaps a verbose flag could be used to allow more debug info to be shown.

[Discord thread for ref](https://discord.com/channels/841451783728529451/1318238008352309279/1318238011200245822)