python: Fix invalid varable/function references.

This code referred to "rows" where it meant to refer to "fetched_rows".
The patch resolves flake8 error:

  F821 undefined name 'rows'

python/build/nroff.py used a function fatal() that was not defined,
which raised the same type of error.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>

python/ovs/db/idl.py

@@ -1221,7 +1221,7 @@ class Transaction(object):
             if len(fetched_rows) != 1:
                 # XXX rate-limit
                 vlog.warn('"select" reply "rows" has %d elements '
-                          'instead of 1' % len(rows))
+                          'instead of 1' % len(fetched_rows))
                 continue
             fetched_row = fetched_rows[0]
             if not Transaction.__check_json_type(fetched_row, (dict,),