ChoiceGroup  i18n.

@sarina please review.
```
#. Translators: 'ChoiceGroup' is an input type and should not be translated.
#: common/lib/capa/capa/inputtypes.py:451
#, python-brace-format
msgid "ChoiceGroup: unexpected tag {tag_name}"
msgstr ""

#: common/lib/capa/capa/inputtypes.py:462
msgid "Answer received."
msgstr ""

#. Translators: '<choice>' is a tag name and should not be translated.
#: common/lib/capa/capa/inputtypes.py:488
#, python-brace-format
msgid "Expected a <choice> tag; got {given_tag} instead"
msgstr ""
```