flatten partial result errors properly

Hello @x-ma, 

Please review the following commits I made in branch 'eford-fix-read-undefined-in-partial-results'.

1a8504cf904f6c2e94b6b60716e62b9d8f0a062d (2014-10-16 16:57:54 -0700)
flatten partial result errors properly

R=@x-ma
VISIBLE_CHANGES=flatten partial result errors properly