RemoteSearch: don't complete a search that was cancelled

This closes a race between setTerms and a slow GetInitialResultSet.
The bug manifests as follows:
- initial search for a short string
- previous results === undefined, call GetInitialResultSet
- user types more, cancel previous search in setTerms()
- mainloop, then _gotResults([])
- previous results === [], !!previous results === true
- therefore call GetSubsearchResultSet with an empty list of results
- _gotResults() from GetSubsearchResultSet is empty
- much later, return from GetInitialResultSet is discarded by
  cancellable
- user unhappy because what he searched for is not there

After this fix, the flow is:
- initial search for a short string
- previous results === undefined, call GetInitialResultSet
- user types more, cancel previous search in setTerms()
- mainloop, but no _gotResults
- previous results === undefined, call GetInitialResultSet again with
  longer string
- some time later, return from first GetInitialResultSet is discarded
  by cancellable
- soon after, return from second GetInitialResultSet comes with good
  results
- user happy

https://bugzilla.gnome.org/show_bug.cgi?id=745861

js/ui/remoteSearch.js

@@ -236,7 +236,9 @@ const RemoteSearchProvider = new Lang.Class({
 
     _getResultsFinished: function(results, error, callback) {
         if (error) {
-            if (!error.matches(Gio.IOErrorEnum, Gio.IOErrorEnum.CANCELLED))
+            if (error.matches(Gio.IOErrorEnum, Gio.IOErrorEnum.CANCELLED))
+                return;
+
             log('Received error from DBus search provider %s: %s'.format(this.id, String(error)));
             callback([]);
             return;