The request for a better error message and better documentation

## For feature request

I'm a plugin/theme author and I appreciate your excellent work on static blogging.

But I find that both usage and API doc hosted on https://hexo.io are very unclear. Due to the dynamic-typed nature of Javascript, I can find little useful information on listed API function signature without being fully documented: I have no idea what argument I should pass, what type this variable is, what property this argument has, what I should return in callback.

Also the examples provided by doc are trivial and not very helpful. I have to dig into source code or program with the power of `console.log`, to get it work.

As for the error output, once I did something wrong I can only get an exception with little useful message thrown from a deeeeeeeeeep `Promise` async call stack. If the error occurs in post rendering I can get some feedback like which line of which file (sometimes not), but I can only get `unknown error` if error occurs in script/plugin execution.

Probably I'm not able to improve error log, but I'm glad to help improving the docs. Where can I start?

中文版：
我是一个Hexo的主题和插件作者，非常感谢你们打造的博客系统。

但是我在深入的使用中发现了文档的一些问题，比如使用说明和API写的非常不清楚。由于Javascript是一门动态类型语言，所以如果不在文档中详细写明的话，从API的函数签名中我是基本找不到任何有用信息的：不知道该传入什么，不知道类型是什么，不知道属性有哪些，不知道回调该返回什么。

除此之外文档给出的例子也是，太简单，起不到什么作用。这时候就只能深入发掘源码，或者是基于`console.log`编程。

至于错误输出，只要一步做错了就只能得到抛出来的异常，一长串的调用栈，基本没有用的错误信息。如果错误是发生在帖子渲染阶段，我倒是能得到是哪个文件第几行的错误输出（也不是总有）；如果错误发生在脚本/插件执行阶段，就只能得到一个`unknown error`。

错误输出我可能是帮不了什么，不过我是很愿意帮助改进文档的。请问该从哪里开始？