Fix using aggregators before aggregation

Summary:
If we register aggregator and immediatelly ask for aggregated value,
previous code was returning initial value, so we have to do the same.

Additionally - cleaning up errors/exceptions to be more understandable
(vs NullPointerEx for example)

Test Plan:
mvn install
AggregatorsBenchmark and ReducersBenchmark

Reviewers: majakabiljo, pavanka, sergey.edunov, maja.kabiljo

Differential Revision: https://reviews.facebook.net/D24951