MRG+1 fixed FeedExporter shutdown log messages

[kmike]

logfmt % "Stored" happens before logger formats the message, so % vs %% needs to be reversed.

This fixes the following error during shutdown:

2015-05-18 19:21:45+0500 [scrapy] ERROR: Error caught on signal handler: <bound method ?.close_spider of <scrapy.extensions.feedexport.FeedExporter object at 0x10c7ea390>>
Traceback (most recent call last):
 File "/Users/kmike/envs/scraping/lib/python2.7/site-packages/twisted/internet/defer.py", line 577, in _runCallbacks
   current.result = callback(current.result, *args, **kw)
 File "/Users/kmike/envs/scraping/lib/python2.7/site-packages/Scrapy-0.25.1-py2.7.egg/scrapy/extensions/feedexport.py", line 187, in <lambda>
   d.addErrback(lambda f: logger.error(logfmt % "Error storing", log_args,
TypeError: format requires a mapping

Export is successful, but error handler is called because there is the same exception in success handler.

To get it try @eliasdorneles's example from here: scrapy/scrapy.org#29 (scrapy runspider example.py -o res.json)

[eliasdorneles]

thanks, @kmike :)