runtime: restore error message on early needm

needm used to print an error before exiting when it was called too
early, but this error was lost in the transition to Go.  Bring back
the error so we don't silently exit(1) when this happens.

Change-Id: I8086932783fd29a337d7dea31b9d6facb64cb5c1
Reviewed-on: https://go-review.googlesource.com/1226
Reviewed-by: Russ Cox <rsc@golang.org>