Warn user if loose files are not enabled.

[reg2k]

By default, Fallout 4 does not load 'loose files' - files in the Data directory that are not within a BA2 archive.

As with most mods, MCM requires that loose file support be enabled with the following INI settings in Fallout4Custom.ini:

[Archive] 
bInvalidateOlderFiles=1
sResourceDataDirsFinal=

MCM's strings (translation files) are not loaded if loose files are not enabled, resulting in pre-translation strings like $MOD_CONFIG appearing in place of Mod Config.

We should consider warning the user if loose file loading is not enabled.