-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.

See merge request !99