Sphinx crashes if unknown po file exists

[tk0miya]

Problem

• Sphinx crashes if unknown po file exists

Procedure to reproduce the problem

$  sphinx-quickstart -q -aa -pp test
$  cd test
$  mkdir -p locales/en/LC_MESSAGES/sphinx
$  touch locales/en/LC_MESSAGES/sphinx/index.po
$  make html SPHINXOPTS='-D language=en'

Error logs / results

# Sphinx version: 2.0.0+
# Python version: 3.6.5 (CPython)
# Docutils version: 0.14
# Jinja2 version: 2.10
# Last messages:
#   loading translations [en]...
#
#   done
#
#   making output directory...
#
#   building [mo]: targets for 1 po files that are out of date
#
#   writing output... [100%] locales/en/LC_MESSAGES/sphinx.mo
#
# Loaded extensions:
#   sphinx.ext.mathjax (2.0.0+) from /Users/tkomiya/work/sphinx/sphinx/ext/mathjax.py
#   alabaster (0.7.10) from /Users/tkomiya/work/tmp/lib/python3.6/site-packages/alabaster-0.7.10-py3.6.egg/alabaster/__init__.py
Traceback (most recent call last):
  File "/Users/tkomiya/work/sphinx/sphinx/cmd/build.py", line 304, in build_main
    app.build(args.force_all, filenames)
  File "/Users/tkomiya/work/sphinx/sphinx/application.py", line 340, in build
    self.builder.compile_update_catalogs()
  File "/Users/tkomiya/work/sphinx/sphinx/builders/__init__.py", line 281, in compile_update_catalogs
    self.compile_catalogs(catalogs, message)
  File "/Users/tkomiya/work/sphinx/sphinx/builders/__init__.py", line 236, in compile_catalogs
    catalog.write_mo(self.config.language)
  File "/Users/tkomiya/work/sphinx/sphinx/util/i18n.py", line 71, in write_mo
    with io.open(self.po_path, 'rt', encoding=self.charset) as file_po:
FileNotFoundError: [Errno 2] No such file or directory: '/Users/tkomiya/work/tmp/doc/test/locales/en/LC_MESSAGES/sphinx.po'

Expected results

Sphinx skips them and goes builds fine.

Environment info

• OS: Mac
• Python version: 3.7.0
• Sphinx version: 2.0.0+

[tk0miya]

Fixed by #5400.