Adding error codes and better logging

[rtetley]

This was initially related to #953.
However the strategies invoked below don't work for the moment.
I opened an issue here: microsoft/language-server-protocol#2069
Aksed a question here: https://stackoverflow.com/questions/79254724/how-to-get-a-partialresulttoken-from-the-textdocument-documentsymbol-request-in
Depending on the response I will also open an issue in the vscode repo.

[rtetley]

I have explored several options:

• First I tried to implement it using a partialResult or workDone tokens (see here). But vscode does not seem to send such tokens alongside the textDocument/documentSymbol request... Even when the server declares supporting the capability... Maybe something needs to be done client side.
• I also tried, after seeing this, to send back a ServerCancelled error code when the document is busy parsing. The client receives the request but never sends a second one...

[rtetley]

I will investigate some more and I am opening this PR anyways since it opens up important features:

• using custom error codes instead of always sending a RequestFailed one.
• giving a document_state (right now only Parsing and Parsed but later Executing and Executed maybe ?)

[rtetley]

Even though this doesn't fix the original problem I think I will merge anyways because it adds interesting functionality. @gares ?