If autodetection failed, give an error message so we don't just get "Failed to create storage with data .."
If autodetection failed, give an error message so we don't just get "Failed
to create storage with data .."

--HG--
branch : HEAD

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