Unstructured response errors from `doInitialize`

[samcowger]

As of lsp-2.6.0.0/lsp-types-2.2.0.0, the doInitialize field of ServerDefinition is a function that evaluates to an IO (Either ResponseError a). If I'm not mistaken, the ResponseError component of that type could/should be a TResponseError 'Method_Initialize.

This would only change the type of the _xdata portion of the error, refining it from its current type of Aeson.Value down to InitializeError, which would be in line with the spec.

[michaelpj]

Yes, this is, sadly, deliberate. I think the reason was that it turned out that HLS was passing around untyped ResponseErrors a lot, and it was going to be a pain at the time to convert them. Obviously that's doing things backwards, and we should just adapt HLS, but I haven't got to it yet. I would definitely like to do this!