--version fails if platex is not installed

[septatrix]

platex is not required if one wants to e.g. only build the english documentation. It would therefore make sense to catch such errors and list the program as not installed.

Traceback (most recent call last):
  File "./build_docs.py", line 802, in <module>
    main()
  File "./build_docs.py", line 741, in main
    version_info()
  File "./build_docs.py", line 626, in version_info
    subprocess.check_output(["platex", "--version"], universal_newlines=True), n=3
  File "/usr/lib/python3.8/subprocess.py", line 411, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.8/subprocess.py", line 489, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.8/subprocess.py", line 854, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.8/subprocess.py", line 1702, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'platex'

[JulienPalard]

Fixed in 3173336, thanks for reporting :)