Fixes #3324.

Robert suggested not reporting errors until the end of the output.
which I'd also like to do, but errPrintedOutput makes that a bigger
change than I want to do before Go 1.  This change should at least
remove the confusion we had.

# Building packages and commands for linux/amd64.
runtime
errors
sync/atomic
unicode
unicode/utf8
math
sync
unicode/utf16
crypto/subtle
io
syscall
hash
crypto
crypto/md5
hash/crc32
crypto/cipher
crypto/hmac
crypto/sha1
go install unicode: copying /tmp/go-build816525784/unicode.a to /home/rsc/g/go/pkg/linux_amd64/unicode.a: short write
hash/adler32
container/list
container/ring
...

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5837054