LSP: Handle case when user tries to include a directory

In case there is a #include "" this is not highlighted, because LSP server crashes while executing on_doc with Sys_error("Is a directory"). Probably get_scope exits by throwing an exception, but ligo info get-scope from CLI works normally and returns

  "errors": [
    {
      "status": "error",
      "stage": "preprocessing",
      "content": {
        "message": "File \"\" not found.",
...

This is also bad because in !2358 (merged) we're saving file contents to hashtabl during on_doc so we can parse CST later, and currently exceptions from get_scope are breaking this data flow.

Current state

get-scope returns an error instead of failing and causing crashes, but error is still not pretty and has no range, because it comes from a caught OCaml exception

Acceptance criteria

• #include "" is correctly highlighted as an error in diagnostics