propagate timeout errors properly in multiplexing cache's mget

Hello @chaosgame, @x-ma, @sboora, @nicks, 

Please review the following commits I made in branch 'eford-fix-thing-maybe'.

d1af96a470914bd37e1f961d48c8af94b61de6f4 (2014-09-19 14:20:25 -0700)
propagate timeout errors properly in multiplexing cache mget

R=@chaosgame
R=@x-ma
R=@sboora
R=@nicks
Can we add the `flattenErrors` functionality to `PartialResultError` directly?  This seems like the only appropriate use-case and a likely error if we forget it.