avoid crashing if mpim directory does not exist in the export

[rusq]

When mpim.json contains a mpim that was empty (did not contain any messages), the directory mpim-....-1 may not exist.

This fix prevents "Internal Server Error" when accessing one of such conversations.

Sample error from the backend:

Traceback (most recent call last):
  File "/Users/rustam/wp/_thirdparty/slack-export-viewer/venv/lib/python3.9/site-packages/werkzeug/serving.py", line 335, in run_wsgi
    execute(self.server.app)
  File "/Users/rustam/wp/_thirdparty/slack-export-viewer/venv/lib/python3.9/site-packages/werkzeug/serving.py", line 322, in execute
    application_iter = app(environ, start_response)
  File "/Users/rustam/wp/_thirdparty/slack-export-viewer/venv/lib/python3.9/site-packages/flask/app.py", line 2548, in __call__
    return self.wsgi_app(environ, start_response)
  File "/Users/rustam/wp/_thirdparty/slack-export-viewer/venv/lib/python3.9/site-packages/flask/app.py", line 2528, in wsgi_app
    response = self.handle_exception(e)
  File "/Users/rustam/wp/_thirdparty/slack-export-viewer/venv/lib/python3.9/site-packages/flask/app.py", line 2525, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/rustam/wp/_thirdparty/slack-export-viewer/venv/lib/python3.9/site-packages/flask/app.py", line 1822, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/rustam/wp/_thirdparty/slack-export-viewer/venv/lib/python3.9/site-packages/flask/app.py", line 1820, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/rustam/wp/_thirdparty/slack-export-viewer/venv/lib/python3.9/site-packages/flask/app.py", line 1796, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "/Users/rustam/wp/_thirdparty/slack-export-viewer/slackviewer/app.py", line 67, in mpim_name
    messages = flask._app_ctx_stack.mpims[name]
KeyError: 'mpdm-foo--bar--baz-1'

[rusq]

@hfaran can I have your opinion on this please, when you have time?

[rusq]

Thank you!