send "failed reading file" message to debug

This message was being logged as an error every time MediaFile failed to parse a
file. But this is not actually an error -- the importer uses FileTypeErrors to
determine whether a file is music or not. This resulted in error logs for every
album art file, .m3u, etc. in the imported directory. Verbose output is a better
home for this message.

beets/library.py

@@ -276,7 +276,7 @@ class Item(object):
         try:
             f = MediaFile(syspath(read_path))
         except Exception:
-            log.error(u'failed reading file: {0}'.format(
+            log.debug(u'failed reading file: {0}'.format(
                 displayable_path(read_path))
             )
             raise