runtime: higher goroutine arg limit, clearer error
runtime: higher goroutine arg limit, clearer error

Fixes issue 591.

R=ken2
CC=golang-dev
http://codereview.appspot.com/4803054
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go