Reply to all requests (sublimelsp#909)

* Reply to all requests

- When there's no request handler, reply with an error with code MethodNotFound
- When there's an exception thrown in the request handler, translate that
  exception to an error response.
- When there's an exception of type Error thrown in the request handler,
  send that error as-is.

Quoting the JSON-RPC 2.0 spec, which LSP is based on:

    When a rpc call is made, the Server MUST reply with a Response,
    except for in the case of Notifications.

Update message for 0.9.6

Update README with our advertised capabilities

Rename some methods to something more aligned with which notification…

…s they send out

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