Add backslash \ to \Exception resolves #244.

Author: lucaboesch

classes/cache_purger.php

@@ -186,7 +186,7 @@ public static function get_coderunner_cache_definition() {
             'gradingcachedefintionnotfound',
             'qtype_coderunner'
         );
-        throw new Exception($error);
+        throw new \Exception($error);
     }
 
 
@@ -202,7 +202,7 @@ public static function get_first_file_store(cache_definition $definition) {
             'gradingcachefilestorenotfound',
             'qtype_coderunner'
         );
-        throw new Exception($error);
+        throw new \Exception($error);
     }