Report missing plugins in pre-build checks

[focustense]

If a profile is set up with certain plugins, and then those plugins are removed from the load order and the app started again, then fallback plugins will be selected for that session, since every NPC must have both a default and face plugin selected.

This behavior doesn't affect the autosave - if the plugin is subsequently re-enabled and the app restarted, without making any explicit changes, then the originally-selected plugins become selected again. However, while in the incomplete state, this can potentially generate confusing warnings if someone tries to build. The actual warning generate will be a FaceModPluginMismatch, which is technically true for that session but not really representative of what's happening.

Since the original selection information has not been lost, the build checker should report a totally different error, explicitly indicating that the plugin is no longer available or enabled, providing a clear course of action to fix that warning, i.e. re-enabling it.