flag: fix bug in handling of booleans on error

Fixes #3869.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6448072

src/pkg/flag/flag.go

@@ -707,7 +707,7 @@ func (f *FlagSet) parseOne() (bool, error) {
 	if fv, ok := flag.Value.(*boolValue); ok { // special case: doesn't need an arg
 		if has_value {
 			if err := fv.Set(value); err != nil {
-				f.failf("invalid boolean value %q for  -%s: %v", value, name, err)
+				return false, f.failf("invalid boolean value %q for  -%s: %v", value, name, err)
 			}
 		} else {
 			fv.Set("true")