Provide a more helpful error if username is misconfigured

[bohendo]

 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: