Fixed "alias_for points to unknown namespace" error handling.
Fixed "alias_for points to unknown namespace" error handling.

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branch : HEAD

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