cmd/vet/all: print all unparseable lines

In my experience, this usually happens when vet panics.
Dumping all unparseable lines should help diagnosis.

Inspired by the trybot failures in CL 40511.

Change-Id: Ib73e8c8b2942832589c3cc5d33ef35fdafe9965a
Reviewed-on: https://go-review.googlesource.com/40508Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/all/main.go

@@ -217,6 +217,7 @@ func (p platform) vet() {
 
 	// Process vet output.
 	scan := bufio.NewScanner(stderr)
+	var parseFailed bool
 NextLine:
 	for scan.Scan() {
 		line := scan.Text()
@@ -235,7 +236,11 @@ NextLine:
 		case 3:
 			file, lineno, msg = fields[0], fields[1], fields[2]
 		default:
-			log.Fatalf("could not parse vet output line:\n%s", line)
+			if !parseFailed {
+				parseFailed = true
+				fmt.Fprintln(os.Stderr, "failed to parse vet output:")
+			}
+			fmt.Println(os.Stderr, line)
 		}
 		msg = strings.TrimSpace(msg)
 
@@ -258,6 +263,10 @@ NextLine:
 		}
 		w[key]--
 	}
+	if parseFailed {
+		atomic.StoreUint32(&failed, 1)
+		return
+	}
 	if scan.Err() != nil {
 		log.Fatalf("failed to scan vet output: %v", scan.Err())
 	}