Show remediation steps and human readable error for Cannot change source virtual machine error

[JenGoldstrich]

If an already existing managed image is attempted to be modified with a new VM, this can lead to a confusing error "cannot change source virtual machine", users often get confused and think this is a plugin bug, we can catch this specific error and return a useful message like

Failed to create Managed Image: Cannot change source virtual machine

This error usually happens when running two parallel builds simultaneously against the name `managed_image_name`, leading to the images conflicting when one build tries to update the source VM of the other