Improve combineErrors

[joyfulmantis]

Currently, combineErrors is pretty terrible. If we get more than one error, we return everything as an InternalError. However, error types have different meanings, and are handled differently depending on the error code by lsp clients. With the current combineErrors, the error code chosen by the plugin is completely ignored if more than one plugin has an error.

There is no easy fix here because we can only return one error. However, we can do things better. Some options would be to see if all the error codes match, and then return that error if so. Another option would be to have a priority of different error codes, so certain errors would be returned over others if we have both.

[michaelpj]

What about doing something like this: consistently log the error for every handler that fails. If they all fail send a single, specific error that says something like "No handler succeeded in responding to this request, please see the log for individual errors"?

[joyfulmantis]

In broad strokes that is kind of already what we do. The problem is that different error codes are treated in different ways by LSP clients, but now if multiple plugins return an error we just send a InternalError, which may not be the best option.

On a related note, now plugins return an InternalError if they can't respond due to failed dependency to a request, and all those responses are logged with Warn (and we return an InternalError ourselves). At a minimum, I think we shouldn't log with Warn, and possibly not return an InternalError either in these situations. We can do that by either changing the type that plugin handlers return to something like (Either (Maybe ResponseError) (MessageParams m)) or by designating a specific custom LSP error code number to be treated as "Not an error, but choosing not to respond" which we then handle more accurately in combineErrors.

[joyfulmantis]

Closed in #3717