Sphinx crashes when the translation catalog contains invalid footnote references

[shimizukawa]

Describe the bug
Normally, building a document with invalid footnote references will not cause a crash.
However, if the translation catalog contains invalid footnote references, it will crash during the write phase.

To Reproduce
Normal case will emit warnings, but will not crash.
index.rst

Hello
========

* a invalid auto-numbered [#]_ ref.
* a invalid named [1]_ ref.

this emits...

...(snip)...
reading sources... [100%] index
test/index.rst:4: WARNING: Too many autonumbered footnote references: only 0 corresponding footnotes available.
test/index.rst:5: WARNING: Unknown target name: "1".
...(snip)...
build succeeded, 2 warnings.

However, invalid footnote references in translation catalog files, sphinx will crash on writing phase.

index2.rst

Hello
========

* this is literal ``[#]_`` ref.
* this is literal ``[1]_`` ref.

index2.po

msgid "this is literal ``[#]_`` ref."
msgstr "this is literal ''[#]_'' ref."

msgid "this is literal ``[#]_`` ref."
msgstr "this is literal ''[1]_'' ref."

In this example, the backticks were mistakenly changed to single quotes in the translation catalog file, resulting in a footnote reference that is not in the source.
If you build with this translation catalog, you will get the following error:

test> sphinx-build -M html . _build -Dlanguage=ja
Running Sphinx v4.0.0+/37fc43a4d
loading translations [ja]... done
building [mo]: targets for 1 po files that are out of date
writing output... [100%] locale/ja/LC_MESSAGES/index2.mo
building [html]: targets for 2 source files that are out of date
updating environment: [new config] 2 added, 0 changed, 0 removed
reading sources... [100%] index2
test/index.rst:4: WARNING: Too many autonumbered footnote references: only 0 corresponding footnotes available.
test/index.rst:5: WARNING: Unknown target name: "1".
test/index2.rst:4: WARNING: inconsistent footnote references in translated message. original: [], translated: ['[#]_']
test/index2.rst:5: WARNING: inconsistent footnote references in translated message. original: [], translated: ['[1]_']
test/index2.rst:5: WARNING: Unknown target name: "1".
looking for now-outdated files... none found
pickling environment... done
checking consistency... C:\Project\sphinx-dev\sphinx-doc-translations\test\index2.rst: WARNING: document isn't included in any toctree
done
preparing documents... done
writing output... [100%] index2
Exception occurred:
  File ".venv\lib\site-packages\docutils\nodes.py", line 625, in __getitem__
    return self.attributes[key]
KeyError: 'refid'

Expected behavior
No crash

Your project
test.zip

Environment info

• OS: Windows, Linux
• Python version: 3.9.0
• Sphinx version: 4.0.0
• Sphinx extensions: nothing
• Extra tools: nothing

Additional context
Actual case

• error: https://readthedocs.org/projects/sphinx-zh-cn/builds/12833891/
• translation catalog: (already fixed) https://www.transifex.com/sphinx-doc/sphinx-doc/translate/#zh_CN/usage--restructuredtext--basics/138613808?q=text%3A'You+can+also+explicitly+number+the+footnotes'
• ReadTheDocs:
  

[shimizukawa]

Thanks!