[API] Add error messages to dispatch API

- Add a error messages to the dispatch
API (https://code.forgejo.org/api/swagger#/repository/DispatchWorkflow)
when incorrect values are given. Otherwise an incorrect error message is
shown to the user.
- Relevant https://codeberg.org/forgejo/forgejo/issues/4765#issuecomment-2125392

(cherry picked from commit db78a3abed)

routers/api/v1/repo/action.go

@@ -621,10 +621,10 @@ func DispatchWorkflow(ctx *context.APIContext) {
 	name := ctx.Params("workflowname")
 
 	if len(opt.Ref) == 0 {
-		ctx.Error(http.StatusBadRequest, "ref", nil)
+		ctx.Error(http.StatusBadRequest, "ref", "ref is empty")
 		return
 	} else if len(name) == 0 {
-		ctx.Error(http.StatusBadRequest, "workflowname", nil)
+		ctx.Error(http.StatusBadRequest, "workflowname", "workflow name is empty")
 		return
 	}