Reject messages sent to groups with no members

If you're the last member of the group, it doesn't make sense to send
messages to it. Previously, we would wait forever for a callback,
causing a stuck pending state indicator.

// FREEBIE

libtextsecure/sendmessage.js

@@ -148,6 +148,9 @@ MessageSender.prototype = {
         timestamp = timestamp || Date.now();
         var me = textsecure.storage.user.getNumber();
         numbers = numbers.filter(function(number) { return number != me; });
+        if (numbers.length === 0) {
+            return Promise.reject(new Error('No other members in the group'));
+        }
 
         return new Promise(function(resolve, reject) {
             this.sendMessageProto(timestamp, numbers, proto, function(res) {