If, as you mention, taiga works perfectly on localhost, the problem is in the proxy pass that you are trying to make with Apache. The browser error indicates that the proxy pass cannot serve the files, probably because Apache is not reaching localhost:9000.
We have an example in the manual that you followed to do what you want using nginx and the example that @jcain proposes looks very good too. Personally we prefer nginx over Apache to act as a reverse proxy, but both should work perfectly if they are properly configured.
