aerc aborting a send or reply when the embedded $EDITOR exits with a non zero status would mimic git's behavior when interactively writing a commit with `git commit'.

to reiterate the git example, when the $EDITOR exits with an error, git will cancel the commit.

for example, when using vim one would could exit the editor with `:cq' to force a non-zero exit status. considering that many users of aerc use vim as their $EDITOR, it does not make sense to (prompt to) send a message if the embedded $EDITOR exits with an error.