FOMOD installer: tell users if the fomod script contains impossible combinations

[Al12rs]

FOMOD can define groups with specific selection requirements, such as ExactlyOne or AtLeastOne etc. It can also define Required option (options can be conditionally required as well), potentially creating impossible combinations such as:

• 2 required options in an ExactlyOne group.

These situations need to be communicated to the user in some way. It's debatable whether the App should allow users to ignore the requirements and install anyways.

[BellCubeDev]

Additional cases:

• NotUsable in a SelectAll group
• all NotUsable options in a SelectExactlyOne or SelectAtLeastOne group
• multiple Required options in a SelectAtMostOne group

[erri120]

Some installers have only one option in a group, but that group is marked as either "exactly one" or "at least one" and the option isn't marked as required. This isn't "impossible" but something where I updated our code to force the single option to be of type "required" even if the original installer doesn't: f01ae9f91f362a0ec70a884534302a20b6f22d97