Presentation: Improve exceptional RPC responses

[grigasp]

At the moment PresentationRpcImpl uses its own system to handle majority of exceptional cases:

• When a timeout occurs, it returns a specific HTTP 200 response, which tells the frontend to repeat the request. Instead, a proper HTTP response should be sent (408?)

• When a request processing error occurs, e.g. a bad argument or something like that, it returns an HTTP 200 response containing the error message. Instead, we should probably return HTTP 400.

[aruniverse]

Fyi @paulius-valiunas @GytisCepk