Implement handlers for language server error responses

According to the LSP standard, certain failures on the server side should be reported by sending an error response to the client. These responses have to be correctly handled by middleware code to prevent displaying a generic Request failed error notification. We should show a custom notification instead with specific error messages.

• rename errors, like new name conflicting with an existing one
• go to definition errors (eg. trying to use this function for an excluded file)