Add missing checks for custom installs

[mmstick]

Ensure that the following works:

• [ ] EFI size is >= 500MiB
• [ ] EFI partition is on a GPT disk
• [ ] EFI partition is FAT32
• [ ] Legacy installs cannot install to GPT
• [ ] Legacy installs cannot install to disks larger than 2TiB
• [ ] EFI installs cannot install to MBR

And that the appropriate error messages / tooltips are provided.