Show an error or warning when a customization isn't supported for an image type

[achilleas-k]

Not all customizations are supported by all image types. While we do check options and produce errors for most of these (see the checkoptions() function in each distro), some are silently ignored and can create confusion (e.g., #2255).

This issue is for identifying all customizations that are silently ignored and adding errors or warnings.

Feel free to edit this issue to add any customizations that are ignored as they are discovered.

• Filesystem customizations for image-installer (not supported; no error or warning)
• Hostname for image-installer (not supported; no error or warning)
• Kernel customizations for image-installer (not supported; no error or warning)