Add Core-Specific Notifications for Missing BIOS

[televandalist]

I imagine it going like this:

• User loads the PSX Beetle core in RALib.
• Before loading a game, RALib detects that the BIOS for that core are missing.
• The message could be something like "BIOS not found! In order to run games using this core, you must have one or more of the following BIOS files in the RALibretro System Directory: scph5500.bin (JP), scph5501.bin (US), and/or scph5502.bin (EU).
• Maybe a hash check for the BIOS that would change the notification to inform the user that they're not named properly.
• It could even be similar to how RetroArch does it in the Desktop Menu: https://i.lensdump.com/i/ie5s6e.png

In any case, I think it would be helpful to people and could cut down on the same question being asked many, many, many times :D

[Jamiras]

With #419, if the core reports a missing BIOS, that information should be displayed for the user. In the case of Beetle PSX, it already generates a giant message through the video output:

I believe RetroArchs BIOS checks are implemented via info files for each core, which RALibretro doesn't currently use.