Do not fail when the runner pane is closed - open other instead

[tscolari]

The removed code was preventing a new pane from being opened again after the old one was closed, in particular when used with the option vim.g['test#preserve_screen'] = 0

Not having the code haven't caused any known issue.

(fixes #229).

[mvanderkamp]

I don't think we should go with this approach. Failing fast and loud is useful. For example see #225. Instead of silencing all error messages, IMO we should fix the problem.

Here are a few ways we could do that:

1. Catch errors when a user has closed the runner on their own instead of using VimuxCloseRunner
2. Always check for the existence of the runner before trying to interact with it (e.g. with hasRunner).
3. Periodically poll for the existence of the runner.

[tscolari]

@mvanderkamp I think what you said makes total sense.
I've pushed another commit attempting to do what you said, it does work in the case I mentioned - although this is my first time writing vimscript, so might not be optimal.