Better signify missing assets on Linux
Currently when the assets are not present the game just sigsegvs. I've noticed that 90% of linux-related tech support questions are "I just compiled the game and it throws a sigsegv on boot". An error message saying the assets were not found would be very helpful for this case.
(this isn't always user error, a lot of people's SRB2WADDIR gets pointed into some fake C:/ drive in wine for some reason???)