Remove uneeded error, fix skip bug

src/lib/Enumerable.vala

@@ -146,7 +146,7 @@ namespace Invercargill {
             throw new SequenceError.NO_ELEMENTS("The sequence contains no elements");
         }
 
-        public virtual T first_or_default() throws SequenceError {
+        public virtual T? first_or_default() {
             var tracker = get_tracker();
             if(tracker.has_next()) {
                 return tracker.get_next();

src/lib/Queries/Skip.vala

@@ -31,6 +31,7 @@ namespace Invercargill {
                     if(!tracker.has_next()) {
                         return false;
                     }
+                    tracker.get_next ();
                     c++;
                 }