fix: Don't print ErrHelp in ParseAll

This is a fixup of #407 to ensure behavior of ExitOnError is consistent.

Author: tomasaschan

flag.go

@@ -1202,6 +1202,9 @@ func (f *FlagSet) ParseAll(arguments []string, fn func(flag *Flag, value string)
 		case ContinueOnError:
 			return err
 		case ExitOnError:
+			if err == ErrHelp {
+				os.Exit(0)
+			}
 			fmt.Fprintln(f.Output(), err)
 			os.Exit(2)
 		case PanicOnError: