code adjustments according to pep8

gallery_dl/exception.py

@@ -6,14 +6,18 @@
 # it under the terms of the GNU General Public License version 2 as
 # published by the Free Software Foundation.
 
+
 class NoExtractorError(Exception):
     """No extractor can handle the given URL"""
 
+
 class AuthenticationError(Exception):
     """Invalid or missing login information"""
 
+
 class AuthorizationError(Exception):
     """Insufficient privileges to access a resource"""
 
+
 class NotFoundError(Exception):
     """Requested resource (gallery/image) does not exist"""