cmd/asm: report more than one instruction encoding error

Also, remove output file if there are encoding errors.
The extra reports are convenient.
Removing the output file is very important.
Noticed while testing.

Change-Id: I0fab17d4078f93c5a0d6d1217d8d9a63ac789696
Reviewed-on: https://go-review.googlesource.com/18845
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/asm/main.go

@@ -47,21 +47,28 @@ func main() {
 	}
 	ctxt.Bso = obj.Binitw(os.Stdout)
 	defer ctxt.Bso.Flush()
-	ctxt.Diag = log.Fatalf
 	output := obj.Binitw(fd)
 	fmt.Fprintf(output, "go object %s %s %s\n", obj.Getgoos(), obj.Getgoarch(), obj.Getgoversion())
 	fmt.Fprintf(output, "!\n")
 
 	lexer := lex.NewLexer(flag.Arg(0), ctxt)
 	parser := asm.NewParser(ctxt, architecture, lexer)
+	diag := false
+	ctxt.Diag = func(format string, args ...interface{}) {
+		diag = true
+		log.Printf(format, args...)
+	}
 	pList := obj.Linknewplist(ctxt)
 	var ok bool
 	pList.Firstpc, ok = parser.Parse()
-	if !ok {
+	if ok {
+		// reports errors to parser.Errorf
+		obj.Writeobjdirect(ctxt, output)
+	}
+	if !ok || diag {
 		log.Printf("asm: assembly of %s failed", flag.Arg(0))
 		os.Remove(*flags.OutputFile)
 		os.Exit(1)
 	}
-	obj.Writeobjdirect(ctxt, output)
 	output.Flush()
 }