Add code_lens_error to JsonRpc

Author: Étienne Lévesque

apps/language_server/lib/language_server/json_rpc.ex

@@ -203,4 +203,5 @@ defmodule ElixirLS.LanguageServer.JsonRpc do
 
   defp error_code_and_message(:request_cancelled), do: {-32800, "Request cancelled"}
   defp error_code_and_message(:content_modified), do: {-32801, "Content modified"}
+  defp error_code_and_message(:code_lens_error), do: {-32900, "Error while building code lenses"}
 end