Make bind error more readable (#17750)

- Add the related fieldNames into the response JSON, such that the developer can figure out what's going on.
- Related:
https://github.com/go-gitea/gitea/issues/17126#issuecomment-937848295

routers/api/v1/api.go

@@ -65,6 +65,7 @@
 package v1
 
 import (
+	"fmt"
 	"net/http"
 	"reflect"
 	"strings"
@@ -551,7 +552,7 @@ func bind(obj interface{}) http.HandlerFunc {
 		var theObj = reflect.New(tp).Interface() // create a new form obj for every request but not use obj directly
 		errs := binding.Bind(ctx.Req, theObj)
 		if len(errs) > 0 {
-			ctx.Error(http.StatusUnprocessableEntity, "validationError", errs[0].Error())
+			ctx.Error(http.StatusUnprocessableEntity, "validationError", fmt.Sprintf("%s: %s", errs[0].FieldNames, errs[0].Error()))
 			return
 		}
 		web.SetForm(ctx, theObj)