Gracefully handles dropped connections.

Closes #3

src/main/java/edu/unl/cse/csce361/socket_chat/Chat.java

@@ -178,12 +178,23 @@ public class Chat {
         boolean myTurnToTalk = isHost;
         try {
             while (!message.equals("EXIT")) {
+                try {
                     if (myTurnToTalk) {
                         message = localInput.readLine();
                         remoteOutput.println(encipher(message));
                     } else {
-                    message = decipher(remoteInput.readLine());
+                        String encipheredMessage = remoteInput.readLine();
+                        if (encipheredMessage != null) {
+                            message = decipher(encipheredMessage);
                             localOutput.println(message);
+                        } else {
+                            localOutput.println("Received null message: lost connection to remote chatter. Terminating.");
+                            message = "EXIT";
+                        }
+                    }
+                } catch (SocketException ignored) {
+                    localOutput.println("Unable to exchange message: lost connection to remote chatter. Terminating.");
+                    message = "EXIT";
                 }
                 myTurnToTalk = !myTurnToTalk;
             }