Improve handling of vanished directories

[phst]

Sometimes files and directories vanish while still open in Emacs. In that case the p4-update check will throw errors such as (file-error "Setting current directory" "no such file or directory" "/foo/bar"). I think this comes from https://github.com/gareth-rees/p4.el/blob/master/p4.el#L1444. I suggest to add a file-accessible-directory-p check before trying to start the process.