We need to show a message when erlang install is incomplete

[lukaszsamson]

    > Please post logs from vscode output panel

After seeing the logs of the vscode output panel about elixir_ls extension, it says a incomplete erlang installation. At the time of writing, I only install elixir by sudo dnf install elixir in my Fedora machine, it may missing some dependencies.
related links:
#231 (comment)
elixir-lang/elixir-lang.github.com#1499
sudo dnf install erlang get a fully complete erlang installation, and the elixir_ls extension works. Many thanks for your prompt tips and works.

Originally posted by @topazus in elixir-lsp/vscode-elixir-ls#271 (comment)