Gracefully handle the editor exiting with a non-zero exit code

[Calamitous]

Suggested by akspecs:

Someone who uses vim can force vim to exit with a non zero exit status
with `:cq'

Perhaps the same is possible with other editors, but I've only tested
this with vim.  When editing a post and choosing to `:cq' out of the
editor (git effectively aborts a commit [--amend / other]), iris will
swallow the post, seemingly without a trace or a deleted message in the
thread it may have used to be a part of.

This may be worth noting, or even a feature -- and not a bug.  But I
figured I'd write you here to see what you think.

I reckon if iris were to acknowledge the exit error of the editor and
effectively abort any edits, that would be a behaviour that syncs up
more with the expected behaviour git.