text: Title: Unexpected exception while updating completions
  Body: <!--- Provide a general summary of the issue in the Title above -->
  When I set $UPDATE_COMPLETIONS_ON_KEYPRESS = True and type for instance /usr/bin/ls -a in terminal, following exception is thrown: "Exception [Errno 13] Permission denied: '/usr/bin/ls.json'"
  <!--- If you have a question along the lines of "How do I do this Bash command in xonsh"
  please first look over the Bash to Xonsh translation guide: https://xon.sh/bash_to_xsh.html
  If you don't find an answer there, please do open an issue! -->
  
  ## xonfig
  
  <details>
  
  ```
  +------------------+----------------------+
  | xonsh            | 0.13.4               |
  | Python           | 3.8.10               |
  | PLY              | 3.11                 |
  | have readline    | True                 |
  | prompt toolkit   | 3.0.36               |
  | shell type       | prompt_toolkit       |
  | history backend  | json                 |
  | pygments         | 2.14.0               |
  | on posix         | True                 |
  | on linux         | True                 |
  | distro           | ubuntu               |
  | on wsl           | False                |
  | on darwin        | False                |
  | on windows       | False                |
  | on cygwin        | False                |
  | on msys2         | False                |
  | is superuser     | False                |
  | default encoding | utf-8                |
  | xonsh encoding   | utf-8                |
  | encoding errors  | surrogateescape      |
  | xontrib          | []                   |
  | RC file 1        | /home/ralis/.xonshrc |
  +------------------+----------------------+
  
  ```
  
  </details>
  
  ## Expected Behavior
  <!--- Tell us what should happen -->
  The warning should be either more subtle or no completion suggestions should be shown.
  ## Current Behavior
  <!--- Tell us what happens instead of the expected behavior -->
  Huge multi-line error is printed.
  <!--- If part of your bug report is a traceback, please first enter debug mode before triggering the error
  To enter debug mode, set the environment variable `XONSH_DEBUG=1` _before_ starting `xonsh`.
  On Linux and OSX, an easy way to to do this is to run `env XONSH_DEBUG=1 xonsh` -->
  
  ### Traceback (if applicable)
  
  <details>
  
  ```
  
  Unhandled exception in event loop:
    File "/home/ralis/.local/lib/python3.8/site-packages/prompt_toolkit/buffer.py", line 1939, in new_coroutine
      await coroutine(*a, **kw)
    File "/home/ralis/.local/lib/python3.8/site-packages/prompt_toolkit/buffer.py", line 1763, in async_completer
      async for completion in async_generator:
    File "/home/ralis/.local/lib/python3.8/site-packages/prompt_toolkit/completion/base.py", line 326, in get_completions_async
      async for completion in completer.get_completions_async(
    File "/home/ralis/.local/lib/python3.8/site-packages/prompt_toolkit/completion/base.py", line 202, in get_completions_async
      for item in self.get_completions(document, complete_event):
    File "/usr/local/lib/python3.8/dist-packages/xonsh/ptk_shell/completer.py", line 58, in get_completions
      completions, plen = self.completer.complete(
    File "/usr/local/lib/python3.8/dist-packages/xonsh/completer.py", line 121, in complete
      return self.complete_from_context(
    File "/usr/local/lib/python3.8/dist-packages/xonsh/completer.py", line 272, in complete_from_context
      for comp in self.generate_completions(
    File "/usr/local/lib/python3.8/dist-packages/xonsh/completer.py", line 233, in generate_completions
      for comp in res:
    File "/usr/local/lib/python3.8/dist-packages/xonsh/completers/man.py", line 137, in completions
      for desc, opts in _parse_man_page_options(cmd).items():
    File "/usr/local/lib/python3.8/dist-packages/xonsh/completers/man.py", line 121, in _parse_man_page_options
      path.write_text(json.dumps(options))
    File "/usr/lib/python3.8/pathlib.py", line 1255, in write_text
      with self.open(mode='w', encoding=encoding, errors=errors) as f:
    File "/usr/lib/python3.8/pathlib.py", line 1222, in open
      return io.open(self, mode, buffering, encoding, errors, newline,
    File "/usr/lib/python3.8/pathlib.py", line 1078, in _opener
      return self._accessor.open(self, flags, mode)
  
  Exception [Errno 13] Permission denied: '/usr/bin/ls.json'
  
  ```
  
  </details>
  
  ## Steps to Reproduce
  <!--- Please try to write out a minimal reproducible snippet to trigger the bug, it will help us fix it! -->
  ```xsh
  $UPDATE_COMPLETIONS_ON_KEYPRESS = True
  /usr/bin/ls -  # exception after typing
  ```
  ## For community
  ⬇️  **Please click the 👍 reaction instead of leaving a `+1` or 👍  comment**
  