Feature: replace error symbol

[axet]

Hello!

Using "." (dot) as error symbol very unintuitive, better to show "!" symbol as error instead of "." and current usage "!" replace with letter which describe the error occurred:

"e" - read error
"s" - size error

...

[dundee]

Gdu tries to be compatible with ncdu (as a kind of category standard), therefore we use the same flags that are used there:

https://github.com/dundee/gdu?tab=readme-ov-file#file-flags

But I understand they can be somehow unintuitive 🤷‍♂️