Add capability to exit a head or object fold with a throw

This allows for all fold functions to throw an exception to exit out of a fold with all dependencies still closed down as expected.

This was previously available for key folds, which was necessary for the folds to work in Riak (as max_results in index queries depends one xiting the fold with an exception).  This change now adds a ct test, and adds support for head folds, object folds (key order) and object folds (sqn order)

src/leveled_penciller.erl

@@ -451,7 +451,7 @@ pcl_fetchkeysbysegment(Pid, StartKey, EndKey, AccFun, InitAcc,
                     {fetch_keys, 
                         StartKey, EndKey, AccFun, InitAcc0, 
                         SegmentList, LastModRange, MaxKeys, 
-                        as_pcl},
+                        by_runner},
                     infinity).
 
 -spec pcl_fetchnextkey(pid(),