StringResponse i18n.

Ticket: [BLD-715](https://edx-wiki.atlassian.net/browse/BLD-715) .

Result for StringResponse:
```
#: common/lib/capa/capa/responsetypes.py:1391
msgid "error"
msgstr ""

#: common/lib/capa/capa/responsetypes.py:1418
msgid "or"
msgstr ""
```
@olmar please review.