Introduce PhabricatorEmptyQueryException

Summary: It's dumb to execute a query which we know will return an empty result.

Test Plan: Looked at comment preview with "11", didn't see "1 = 0" in DarkConsole.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5177

src/applications/files/query/PhabricatorFileQuery.php

@@ -22,7 +22,7 @@ final class PhabricatorFileQuery
     return $this;
   }
 
-  public function loadPage() {
+  protected function loadPage() {
     $table = new PhabricatorFile();
     $conn_r = $table->establishConnection('r');