cmds: catch when the editor is misconfigured

Show an error message instead of raising an exception when the
configured editor does not exist.

This fixes https://bugzilla.redhat.com/show_bug.cgi?id=886826

Reported-by: Srinivasa Nallapati <srinivasa.nallapati@walkingtree.in>
Helped-by: Christopher Meng <cickumqt@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>