Use more verbose exception catch.

letsencrypt/le_util.py

@@ -70,7 +70,7 @@ def unique_file(path, mode=0o777):
         try:
             file_d = os.open(fname, os.O_CREAT | os.O_EXCL | os.O_RDWR, mode)
             return os.fdopen(file_d, "w"), fname
-        except OSError, e:
+        except OSError as exception:
             # "File exists," is okay, try a different name.
             if exception.errno != errno.EEXIST:
                 raise