client: only return ErrConnectionFailed on fatal network errors
client: only return ErrConnectionFailed on fatal network errors

Fixes #347.

Signed-off-by: Tamir Duberstein <c96a05aaafbe4b3abc1841a4083a1b0f402c2ba6@gmail.com>

vdemeester/engine-api,docker/engine-api,jwhonce/engine-api,thaJeztah/engine-api