Make a better interface for informing users of syntax errors inline

right now the caret turns red. That might be sufficient, but it's probably not a strong enough indicator.

remember: we don't want the user to get overwhelmed because they keep making syntax errors while they're typing. We want to assist them, not shout errors at them loudly.

首先定位编辑器检测到语法错误时将插入符变为红色的代码。检查现有的内联错误行为以及任何相关的界面入口；最终应确定一种明确且不会让用户感到不知所措的方式，在用户输入时告知他们存在语法错误。

由索引模型根据 Issue 内容生成。