Add a chapter about errors

One concept that I still find quite hard to grasp are Nushell's errors.
As far as I can tell, they behave like raised exceptions in a language like Python.
They bubble up, but can be caught with try or do -i.
However, I haven't seen that spelled out yet.

I see that nushell errors are also created if external commands exit with a non-zero exit code.
But also that isn't mentioned in the book.

I would be happy to add a chapter about errors in Nushell if someone could me guide through the things I am unsure about.

Topics to cover:

• behavior of errors
• try
• do
• error make
• complete

Issue 中没有列出文件、测试或入口点。首先查阅 Nushell 关于错误、try、do -i、error make 和 complete 的文档，然后确认外部命令的非零退出代码的行为；完成标准是添加一个清晰解释每个所列主题的章节。