This works with systems defined by the error delegate function (and it
shows which system is used, and which buffer supplies the “errors”).
When the Emacs next-error system is used and the next-error buffer is a
Spacemacs GNE buffer, it also shows current number / total number.
Implement a delegate function that decides which system to use. Also
check for the visibility of any next-error valid buffer, not just
compilation buffers.
