runtime: say 'fatal error' instead of 'throw'
runtime: say 'fatal error' instead of 'throw'

Fixes issue 4597.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7032043

dsriram/golang,dsriram/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,gnanderson/go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang