* Belgian minister issues Greek exit warning * Large Spanish region says needs financing help * Bankia set to ask for 15 bln euros from Spanish state * French banks make Greek contingency plans -sources By Fiona Ortiz and Nicholas Vinocur May 25 (Reuters) - Central banks and companies risk making a grave error if they do not brace for a possible Greek exit from the euro zone, Belgium's foreign minister said on Friday, rattling markets already alarmed by Spain's deterioratin