The whicherrs query mode takes the position of an error and returns the set of constants, globals and types visible from within the scope of the error being queried.
It is meant to be used as a shortcut to find out which errors should be handled for a given functions call.
LGTM=adonovan
R=golang-codereviews, dominik.honnef, adonovan
CC=golang-codereviews
https://golang.org/cl/167420043
