cmd/go: make build errors more visible

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

src/cmd/go/build.go

@@ -604,7 +604,12 @@ func (b *builder) do(root *action) {
 }
 
 // build is the action for building a single package or command.
-func (b *builder) build(a *action) error {
+func (b *builder) build(a *action) (err error) {
+	defer func() {
+		if err != nil {
+			err = fmt.Errorf("go build %s: %v", a.p.ImportPath, err)
+		}
+	}()
 	if buildN {
 		// In -n mode, print a banner between packages.
 		// The banner is five lines so that when changes to
@@ -753,7 +758,12 @@ func (b *builder) build(a *action) error {
 }
 
 // install is the action for installing a single package or executable.
-func (b *builder) install(a *action) error {
+func (b *builder) install(a *action) (err error) {
+	defer func() {
+		if err != nil {
+			err = fmt.Errorf("go install %s: %v", a.p.ImportPath, err)
+		}
+	}()
 	a1 := a.deps[0]
 	perm := os.FileMode(0666)
 	if a1.link {
@@ -874,7 +884,7 @@ func (b *builder) copyFile(a *action, dst, src string, perm os.FileMode) error {
 	df.Close()
 	if err != nil {
 		os.Remove(dst)
-		return err
+		return fmt.Errorf("copying %s to %s: %v", src, dst, err)
 	}
 	return nil
 }

src/cmd/go/clean.go

@@ -110,7 +110,7 @@ func clean(p *Package) {
 	}
 	dirs, err := ioutil.ReadDir(p.Dir)
 	if err != nil {
-		errorf("%v", err)
+		errorf("go clean %s: %v", p.Dir, err)
 		return
 	}