extract-ofp-errors: Make Python 3 compatible.

extract-ofp-errors: Make Python 3 compatible.

extract-ofp-errors doesn't work with python 3 for the
following reasons:

- several "SyntaxError: invalid syntax":
  print not a keyword anymore. As a function it requires '()'
- AttributeError: 'dict' object has no attribute 'itervalues'
  Use values() instead.

Test done:
Generate using ofp-errors.inc as a reference
Patch for python 3, then regenerate ofp-errors.inc
Diff between the two outputs.

Signed-off-by: Damien Millescamps <damien.millescamps@6wind.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>