insertMethod.java

@ApiMethod(name = "insertQuote",  scopes = {Constants.EMAIL_SCOPE},
	clientIds = {Constants.WEB_CLIENT_ID, 
		     Constants.ANDROID_CLIENT_ID, 
		     com.google.api.server.spi.Constant.API_EXPLORER_CLIENT_ID},
		     audiences = {Constants.ANDROID_AUDIENCE})
	
public Quote insertQuote(Quote quote, User user) throws UnauthorizedException {
	if (user == null) throw new UnauthorizedException("User is Not Valid");
	
	PersistenceManager mgr = getPersistenceManager();
	try {
		if (quote.getId() != null) {
			if (containsQuote(quote)) {
				throw new EntityExistsException("Object already exists");
			}
		}
		mgr.makePersistent(quote);
	} finally {
		mgr.close();
	}
	return quote;
}

Exception handling from google has been updated to OAuthRequestException as mentioned here sir .
Although your technique is much more friendly .
https://cloud.google.com/appengine/docs/java/endpoints/auth
Thanks for the amazing write up its been really helpful.