Psub cleanup not happening in event handlers

[faho]

As reported by @marcusatiliusregulus on gitter:

function f --on-event fish_prompt
    source (echo "echo hello world" | psub)
end

causes psub to not clean up after itself - the "on-job-exit caller" event is never run. This causes issues with pyenv, because that still uses source (thing | psub) internally, and so it will lead to a lot of files in $TMPDIR.

This is caused by f7e2e7d forbidding job handlers inside event handlers from running. I'm going to soften it to still allow caller-exit handlers.