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

No files, tests, or entry points are named in the issue. Start by reviewing Nushell's documentation for errors, try, do -i, error make, and complete, then confirm how external non-zero exit codes behave; done means adding a chapter that clearly explains each listed topic.