Check for errors in system calls

[mvanderkamp]

Problem

When Vimux fails because it tried to do something that's incompatible with the current tmux version, it fails mysteriously. This is because we don't check whether the system calls to tmux succeeded.

Solution

Check system calls by checking v:shell_error. The variable has been around since at least vim version 7.0001.

I've also documented the VimuxDebug option in the help page.