Crash if unable to access BIOS/VBIOS file

[SoftwareGuy]

If Tiny386 is started and it cannot access the BIOS or VBIOS files, it will bomb out with a segmentation fault. This can be triggered if your configuration file points to invalid location for either file.

I believe this is an oversight as the emulator should gracefully throw an error message and then quit without crashing. I suspect this might also affect the ESP32 port; could we output a "NO BIOS" text message on the screen to tell the user they messed up?

[hchunhui]

Indeed. As a prototype there are still many things to do.