Change error message when caching fails

see also: http://stackoverflow.com/questions/27722349/less-js-error-msg-failed-to-save/27750286

lib/less-browser/cache.js

@@ -16,7 +16,7 @@ module.exports = function(window, options, logger) {
                     cache.setItem(path + ':timestamp', lastModified);
                 } catch(e) {
                     //TODO - could do with adding more robust error handling
-                    logger.error('failed to save');
+                    logger.error('failed to save "' + path + '" to local storage for caching.');
                 }
             }
         },