better error messages when mistyping a cloud instance name

EdgeDB CLI Version: 3.1.0-dev (`f4fc73eac35426bc119afa0a1b42c31235c08f53`, master as of 2023-05-22)

if I mistype my instance name or username when trying to connect, I get an error that's not very helpful in figuring out what I did wrong:

```
$ edgedb -I zackelan/notmyinstance
edgedb error: ClientConnectionFailedError: failed to lookup address information: Name or service not known
$ edgedb -I notmyorg/notmyinstance
edgedb error: ClientConnectionFailedError: failed to lookup address information: Name or service not known
```

also when trying to delete the instance:

```
$ edgedb instance destroy -I zackelan/notmyinstance
Do you really want to delete instance "zackelan/notmyinstance"? (type `Yes`)
> Yes
edgedb error: Could not destroy EdgeDB Cloud instance: HTTP error: [404] Resource not found.

$ edgedb instance destroy -I notmyorg/notmyinstance
Do you really want to delete instance "notmyorg/notmyinstance"? (type `Yes`)
> Yes
edgedb error: Could not destroy EdgeDB Cloud instance: HTTP error: [404] Resource not found.
```

on the other hand, `instance status` gives me a much better error:

```
$ edgedb instance status -I zackelan/notmyinstance
edgedb error: instance not found
$ edgedb instance status -I notmyorg/notmyinstance
edgedb error: instance not found
```