Feature: Clear formatter error logs

Code formatters can fail when the code they're formatting is not valid. For example, "black" fails on invalid Python. It would be great if there were a feature that saved formatter errors for pretty display to the end user; it seems like these error messages are discarded at present (see )

I've implemented my own Vim formatter plugin a while ago that provides a version of this, but I'd like to migrate to your general language server for formatting if possible. The error logging feature is an extremely nice to have, so I'm opening this issue in case anyone has the time / inclination to implement something helpful (custom action show stderr on failed formatters, etc). I think formatter debugging capabilities is a fundamental part of a tool that provides code formatting.

Reference:

No repository files, tests, or entry points are named. Review the linked diagnostic-languageserver pull request and the referenced vim-filetype-formatter documentation first; done should mean formatter stderr is retained and made available for useful display after a formatting failure.

Written by the indexing model from the issue text.