GLX: indirect handling fixes

Two changes here. The first slightly simplifies glXImportContextEXT to avoid pointlessly generating an error on the client side which the server will generate for us just fine. The second adjusts the error handling in the legacy CreateContext paths to generate the expected BadValue for unsupported indirect contexts, instead of what we were doing which is raising a GLXBadContext from a GLXIsDirect request that we'd emitted behind the scenes (and thus which doesn't obviously correspond to anything in the client code, making it hard to diagnose).