Provide meaningful warnings for otp configuration issues in VS Code

[mcesaro]

I frequently work on the same project on 2 different systems (one is linux and one is mac os) using the erlang ls extension for VS Code. I keep 2 different otp_path keys configured in the erlang_ls.config file, one for linux and one for mac os, and i manually edit it when I switch my working system. Sometimes, especially when pulling from a git repo, I forget to do the manual switch and there is no warning from erlang_ls of the wrong path. It would be nice to include a warning about the invalid otp path (maybe in the output or the problems tab of VS Code) when the server is launched or re-launched.