-warp checking for invalid map names
I've noticed a bunch of new people getting the "Cannot warp to map 0 (out of range)" error when testing their custom maps. On asking what map name they used, it was clear they didn't use a MAPxx name at all. Sadly it was not obvious to them that other kind of map names are not accepted by SRB2, so I thought it best we make that more clear in-game.
SRB2 now gives you the error "Cannot warp to map [your input] (invalid map name)" if you used -warp with a param that is neither a MAPxx name or a plain number.
