Throw exceptions from Future resolveAll()

Summary: FutureIterator::resolveAll() doesn't throw exceptions because next() only gets a next future without resolving it.

Test Plan:

Futures($futures)->resolveAll();

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin, jungejason

Differential Revision: https://secure.phabricator.com/D2480