Aha! Finally found a reliable way to detect jsonp loading errors.

Before, afterquery would just stop working if your jsonp data source didn't
load for any reason (eg. a syntax error in the input file, like if it's not
really jsonp).  Now we load a *second* <script> file after the first one;
that file always runs, but only after the first one has finished for any
reason (successfully or unsuccessfully).  Since when it succeeds, it calls
successfunc, we need only check whether successfunc has been called to see
if we have a failure.  No stupid timeouts!