* Check errors state on all places of its using (throw exceptions or show a warnings message)
+ To backward compatibility, save "thumbnails_format" value to the config
+ deep_diff() function is extended for the case of "thumbnails" comparison
+ Added unit tests to:
* check a load of configuration for "thumbnails" and "thumbnails_format" options
* check return values for make_and_check_thumbnail_list() function
