Local police are issuing thin ice warnings after two incidents in two days.  Wednesday, three fishermen on Metacomet Lake in Belchertown fell through the ice, but were rescued.  Thursday afternoon, two high school students crashed through the ice on Nashawannuck Pond in Easthampton.  They made it back to shore, but were transported to Cooley Dickinson Hospital as a precaution.  Police say the boys are lucky to be alive.