regresssion in sylvactl user feedback when using a wrong image_key

To workaround #1849 (closed) we merged !3278 (merged).

This has a side-effect of degrading user experience in the (rather frequent) situation where users use image_key with an image key that isn't actually defined (either not existing at all, or for a non enabled image, or in the capm3 case not served by the mgmt cluster).

We ideally would want to solve this for Sylva 1.2.