Revert UsageOnMissing.

This was breaking UsageOnMissing() when combined with validation of
positional arguments, in that it was allowing positional arguments to be
omitted. It also broke UsageOnMissing() somehow so that it did not work.

kong.go

@@ -47,14 +47,13 @@ type Kong struct {
 	resolvers []Resolver
 	registry  *Registry
 
-	noDefaultHelp  bool
-	usageOnError   bool
-	usageOnMissing bool
-	help           HelpPrinter
-	helpFormatter  HelpValueFormatter
-	helpOptions    HelpOptions
-	helpFlag       *Flag
-	vars           Vars
+	noDefaultHelp bool
+	usageOnError  bool
+	help          HelpPrinter
+	helpFormatter HelpValueFormatter
+	helpOptions   HelpOptions
+	helpFlag      *Flag
+	vars          Vars
 
 	// Set temporarily by Options. These are applied after build().
 	postBuildOptions []Option
@@ -219,10 +218,7 @@ func (k *Kong) Parse(args []string) (ctx *Context, err error) {
 		return nil, &ParseError{error: err, Context: ctx}
 	}
 	if err = ctx.Validate(); err != nil {
-		if !k.usageOnError || !isMissingChildError(err) {
-			return nil, &ParseError{error: err, Context: ctx}
-		}
-		ctx.Error = err
+		return nil, &ParseError{error: err, Context: ctx}
 	}
 	if err = k.applyHook(ctx, "AfterApply"); err != nil {
 		return nil, &ParseError{error: err, Context: ctx}