core/fetch: don't log cancelled /get-blocks reqs
core/fetch: don't log cancelled /get-blocks reqs

We had a check to suppress the error when a /get-blocks request
surpassed its deadline, but the error returned from rpc.Call changed
when we switched from ctxhttp to using Go 1.7 with the stdlib context.

PTAL @ryandotsmith 

Closes chain/chainprv#2034

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain