b4: Remove temporary directory on "missing mbox" error

As of 30defdb (b4: Clean up temporary directories by default,
2020-09-27), piem-b4-am-from-mid is supposed to clean up its temporary
directory unless piem-b4-keep-temp-directory is non-nil.  That commit,
however, missed an error case where the cleanup function needs to be
triggered.

Message-Id: <20201025192111.27439-1-kyle@kyleam.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/piem-b4.el b/piem-b4.el
index 31bf6f1..27ecf19 100644
--- a/piem-b4.el
+++ b/piem-b4.el
@@ -101,6 +101,8 @@ This is intended to be used for debugging purposes.")
                  mbox-cover)
             (if (file-exists-p mbox-am)
                 mbox-am
+              (when clean-fn
+                (funcall clean-fn))
               (error "Expected mbox file does not exist: %s" mbox-am))
             clean-fn))))