Data path verification

[historic_bruno]

Currently we assume once the game starts, all expected paths and data exist and have correct permissions. If not, errors can and do occur almost anywhere but the user is not immediately notified. Usually the first noticeable error is:

unifont.cpp(212): Function call failed: return value was -110100 (VFS directory not found)

which still isn't very informative.

Instead we should check at startup that required data paths exist, writable data paths are actually writable, the public mod has been mounted, maybe even that certain required VFS directories and files exist. If a problem is found, the user should be nicely informed that they haven't set up or installed the game correctly, and directed somewhere that will help them.