gc: better error message when ~ operator is found
gc: better error message when ~ operator is found

Fixes issue 844.

R=ken2, rsc, ken3
CC=golang-dev
http://codereview.appspot.com/1593041

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