New get_error_response function.

It returns the Error_Response component of the variant, if the response
happens to be from an error, otherwise the function throws an exception.
Added some support code to the client plugin.

The still starts the engine at 1000x1000 resolution then quits after one
second.
The make_params and make_params_* functions should be in the client namespace.
Moved the Client structure to the new support.(h|cpp) file pair.
make_params and make_params_* are now in support.h.
Added a function that sends a log request to the server.
Pong plugin code refactoring.

New code logs the success or failure after attempting to start the
engine. I moved one comment also, I think.