Do not run parent node on missing child err with usageOnMissing option

Refactors usageOnMissing option to not run parent commands after
printing usage when a child node is missing.

Signed-off-by: hasheddan <georgedanielmangum@gmail.com>

error.go

@@ -1,5 +1,10 @@
 package kong
 
+import (
+	"fmt"
+	"strings"
+)
+
 // ParseError is the error type returned by Kong.Parse().
 //
 // It contains the parse Context that triggered the error.
@@ -10,3 +15,29 @@ type ParseError struct {
 
 // Cause returns the original cause of the error.
 func (p *ParseError) Cause() error { return p.error }
+
+type missingChildError struct {
+	missing []string
+}
+
+func (m *missingChildError) Error() string {
+	if len(m.missing) > 5 {
+		m.missing = append(m.missing[:5], "...")
+	}
+	if len(m.missing) == 1 {
+		return fmt.Sprintf("expected %s", m.missing[0])
+	}
+	return fmt.Sprintf("expected one of %s", strings.Join(m.missing, ",  "))
+}
+
+func newMissingChildError(missing []string) *missingChildError {
+	return &missingChildError{missing}
+}
+
+func isMissingChildError(err error) bool {
+	if err == nil {
+		return false
+	}
+	_, ok := err.(*missingChildError)
+	return ok
+}