Implement client-specific error structures

[ionut-arm]

The initial client returns raw ResponseStatuses as error values for all its methods. A better solution would be to return a client-specific error type which would make a distinction between errors originating in the service and errors raised by the client itself.

This should be done before #4

[ionut-arm]

The conversions happening implicitly within the RequestHandler should be modified to point to errors belonging to the client, not to the service