fetch: Allow skipping undefined URL fetches un multi-core fetch rules

[lggomez]

Currently libretro-fetch if invoked with a multi-core list ends if one of the cores could not be fetched - i.e: when I tried the following and blastem failed to fetch the script ended there and promptly started the build of the previous 3 cores:

./libretro-fetch.sh mednafen_psx_hw mednafen_supafaust mednafen_gba blastem bsnes_mercury citra_canary desmume dosbox_core easyrpg fbalpha2012_neogeo flycast gearboy mupen64plus_next pcsx2

This PR adds a check for an user-definable variable SKIP_UNKNOWN_URL_FETCH which allows to skip each core instead of terminating the script upon URL retrieval errors