Improve error message for connection errors

[TreeOfLearning]

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