Searching before building index triggers PHP error

I've just started testing out the Xapian module tonight. I tried doing a search before building an index, and it spit out an ugly error saying, "Invalid argument supplied for foreach() in /var/www/xapian57/modules/node/node.module on line 930." This is because when xapian_query() runs into an error condition, it returns NULL, which ultimately gets passed to the node module which expects an array.

The attached patch hides the error from normal users, and informs search administrators that something is misconfigured telling them they should check the watchdog log.