gc: remove duplicate errors, give better error for I.(T)
gc: remove duplicate errors, give better error for I.(T)

R=ken2
CC=golang-dev
http://codereview.appspot.com/370041

tianon/debian-golang,gnanderson/go,ssrl/go,dsriram/golang,apparentlymart/golang,tianon/debian-golang,tianon/debian-golang,bdwalton/debian-golang,pombredanne/go-deleteme,dsriram/golang,nightlyone/golang,pombredanne/go-deleteme,bdwalton/debian-golang,tianon/debian-golang,pombredanne/go-deleteme,apparentlymart/golang,bdwalton/debian-golang,ssrl/go,h8liu/golang,gnanderson/go,tianon/debian-golang,h8liu/golang,ssrl/go,bdwalton/debian-golang,ssrl/go,dsriram/golang,tv42/old-go,dsriram/golang,h8liu/golang,tv42/old-go,h8liu/golang,tianon/debian-golang,gnanderson/go,tv42/old-go,tv42/old-go,bdwalton/debian-golang,pombredanne/go-deleteme,ssrl/go,ssrl/go,nightlyone/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,nightlyone/golang,nightlyone/golang,nightlyone/golang,dsriram/golang,bdwalton/debian-golang,bdwalton/debian-golang,h8liu/golang,apparentlymart/golang,bdwalton/debian-golang,ssrl/go,apparentlymart/golang,tv42/old-go,dsriram/golang,gnanderson/go,nightlyone/golang,pombredanne/go-deleteme,tianon/debian-golang,dsriram/golang,tv42/old-go,nightlyone/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,tianon/debian-golang,gnanderson/go,tv42/old-go,h8liu/golang,h8liu/golang