If no error is set, give "BUG: Unknown error" rather than try to print NULL which crashes in some systems.
If no error is set, give "BUG: Unknown error" rather than try to print NULL
which crashes in some systems.

--HG--
branch : HEAD

jkerihuel/dovecot,dscho/dovecot,dscho/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,dscho/dovecot,jkerihuel/dovecot,dscho/dovecot,dscho/dovecot