Closes
https://linear.app/zed-industries/issue/Z-2402/elixir-ls-requires-a-system-elixir-runtime-to-be-installed
Closes
https://linear.app/zed-industries/issue/Z-666/notify-users-that-they-need-go-installed-to-use-go-language-server

Release Notes:

- Added a notification when unable to download the Go language server
because Go is not installed.
- Added a notification when unable to run the Elixir language server
because Elixir is not installed.