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
We should have some way of propagating the error down.  Simply logging it here won't allow us to check outside of `zcache` whether or not it was a timeout.  Look at the `_logUnlessRedisTimeoutError`, `_isPartialResultError`, and `_onlyTimeoutInPartialError` functions in `CacheWrapper.js`