add quick example

tools/goctl/model/sql/gen/error.go

@@ -2,6 +2,4 @@ package gen
 
 import "errors"
 
-var (
-	ErrCircleQuery = errors.New("circle query with other fields")
-)
+var ErrCircleQuery = errors.New("circle query with other fields")

tools/goctl/model/sql/gen/gen.go

@@ -82,7 +82,7 @@ func (g *defaultGenerator) Start(withCache bool) error {
 		}
 	}
 	// generate error file
-	filename := filepath.Join(dirAbs, "error.go")
+	filename := filepath.Join(dirAbs, "vars.go")
 	if !util.FileExists(filename) {
 		err = ioutil.WriteFile(filename, []byte(template.Error), os.ModePerm)
 		if err != nil {