Provide a more helpful error if username is misconfigured

```
Creating new cloudexec- bucket...
Failed to create bucket 'cloudexec-': InvalidRequest: Malformed request
status code: 400, request id: , host id:
```

This error should say something more like "username cannot be empty, reconfigure cloudexec with `cloudexec config` or edit the file at ~/.config/cloudexec/config.toml manually"

Better username validation will also help us detect & debug difficult-to-reproduce edge-cases. For example, some users have reported that setting the username to `cloudexec-myname` results in a bucket creation error:

```
Failed to create bucket 'cloudexec-cloudexec-myname': InternalError: Error processing request
status code: 500, request id: , host id:
```