cmd/doc: continue searching after error reading directory

If a directory in GOPATH is unreadable, we should keep looking for other
packages. Otherwise we can give the misleading error "no buildable Go
source files".

Fixes #16240

Change-Id: I38e1037f56ec463d3c141f0508fb74211cb90f13
Reviewed-on: https://go-review.googlesource.com/31713
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/doc/dirs.go

@@ -77,14 +77,14 @@ func (d *Dirs) bfsWalkRoot(root string) {
 		for _, dir := range this {
 			fd, err := os.Open(dir)
 			if err != nil {
-				log.Printf("error opening %s: %v", dir, err)
-				return // TODO? There may be entry before the error.
+				log.Print(err)
+				continue
 			}
 			entries, err := fd.Readdir(0)
 			fd.Close()
 			if err != nil {
-				log.Printf("error reading %s: %v", dir, err)
-				return // TODO? There may be entry before the error.
+				log.Print(err)
+				continue
 			}
 			hasGoFiles := false
 			for _, entry := range entries {