Improved error message for missing required flags: now list all missi…

…ng required flags for better user guidance. (#340)

Issue: #215

app.go

@@ -464,14 +464,18 @@ func (a *Application) validateRequired(context *ParseContext) error {
 	}
 
 	// Check required flags and set defaults.
+	var missingFlags []string
 	for _, flag := range context.flags.long {
 		if flagElements[flag.name] == nil {
 			// Check required flags were provided.
 			if flag.needsValue() {
-				return fmt.Errorf("required flag --%s not provided", flag.name)
+				missingFlags = append(missingFlags, fmt.Sprintf("'--%s'", flag.name))
 			}
 		}
 	}
+	if len(missingFlags) != 0 {
+		return fmt.Errorf("required flag(s) %s not provided", strings.Join(missingFlags, ", "))
+	}
 
 	for _, arg := range context.arguments.args {
 		if argElements[arg.name] == nil {