Clear context error on post creation when create_at is set (#4615)

api/post.go

@@ -134,6 +134,7 @@ func CreatePost(c *Context, post *model.Post, triggerWebhooks bool) (*model.Post
 
 	if post.CreateAt != 0 && !HasPermissionToContext(c, model.PERMISSION_MANAGE_SYSTEM) {
 		post.CreateAt = 0
+		c.Err = nil
 	}
 
 	post.Hashtags, _ = model.ParseHashtags(post.Message)