Be nicer when emacsclient isn't found

[plexus]

We shell out to emacsclient for a few different things, for finding the version of cider and clj-refactor, as well as for sending a connect sequence to CIDER. For the middleware versions we have fallback values, so e.g. if you are using Calva you can still use --cider-nrepl, but you can't use --emacs, because the connect will throw an exception.

It doesn't really make sense to use --emacs unless you're actually using emacs, but it would still be nice to give a nicer message about what is wrong, or to print a warning and skip the connect, but still run nREPL as usual.