CustomResponse i18n.

Ticket: [BLD-714](https://edx-wiki.atlassian.net/browse/BLD-714) .

Result for `CustomResponse`:
```
#: common/lib/capa/capa/responsetypes.py:1519
#, python-brace-format
msgid ""
"[courseware.capa.responsetypes.customresponse] error getting student answer from {student_answers}\n"
" idset = {idset}, error = {err}"
msgstr ""

#: common/lib/capa/capa/responsetypes.py:1540
msgid "No answer entered!"
msgstr ""

#: common/lib/capa/capa/responsetypes.py:1683
msgid "CustomResponse: check function returned an invalid dictionary!"
msgstr ""
```

@olmar please review.