LSP: notify the user when the language server has crashed

[aktau]

• nvim --version: NVIM v0.5.0-583-g7b529e791 / LuaJIT 2.1.0-beta3
• language server name/version: efm-langserver (mattn/efm-langserver@0b64dd8)
• Operating system/version: Linux Debian (Buster)

LSP log: https://gist.github.com/aktau/1c1ef881c0afb41d9808a821263ea1d7

Steps to reproduce using nvim -u NORC

This basically applies to any language server that can crash. In my case it's due to mattn/efm-langserver#57. A few times now when trying to resolve other issues with nvim-lsp I was confused because the language server had just crashed.

Actual behaviour

When the language server crashes I get no notification that it happened, and am left to wonder why I get no diagnostics or anything else anymore.

Expected behaviour

Either:

1. A notification that the language server has crashed. Or,
2. A notification that the language server has crashed (to allow filing bugs for the language server owner) followed by an automatic restart of the language server. (In my case, this would be preferable, but I'm not sure if this is the case for every language server. It could be a configurable option.)

It would be helpful (but not required), to add a note to the notification such that the user can file a better bug report (with logs) to the langserver.

[teto]

I dont think this should be a regular check but a buffer specific checkhealth could check for a process with the server name etc.

[tjdevries]

First guess is we could do this on_exit for the job. I can try and check this out at some point.