[REQ] Improve description for missing required properties errors in Go Models

Is your feature request related to a problem? Please describe.

Missing required properties error only returns the required property name which can be time consuming to find for large and complex specs.

for _, requiredProperty := range(requiredProperties) {
		if _, exists := allProperties[requiredProperty]; !exists {
			return fmt.Errorf("no value given for required property %v", requiredProperty)
		}
	}

Describe the solution you'd like

Including the class name in the error message would be very helpful by modifying the Go Model template model_simple.mustache

for _, requiredProperty := range(requiredProperties) {
		{{#useDefaultValuesForRequiredVars}}
		if value, exists := allProperties[requiredProperty]; !exists || value == "" {
			if _, ok := defaultValueFuncMap[requiredProperty]; ok {
				allProperties[requiredProperty] = defaultValueFuncMap[requiredProperty]()
				defaultValueApplied = true
			}
		}
		if value, exists := allProperties[requiredProperty]; !exists || value == ""{
		{{/useDefaultValuesForRequiredVars}}
		{{^useDefaultValuesForRequiredVars}}
		if _, exists := allProperties[requiredProperty]; !exists {
		{{/useDefaultValuesForRequiredVars}}
+			return fmt.Errorf("no value given for required property %v in {{ classname }}", requiredProperty)
-			return fmt.Errorf("no value given for required property %v", requiredProperty)
		}
	}

Describe alternatives you've considered

Additional context

Start in modules/openapi-generator/src/main/resources/go/model_simple.mustache and inspect the required-property validation shown in the issue. Update the generated error wording so it includes the model class name, then verify that generated Go models report both the missing property and its class.