export: Improve error message for missing registrations.
export: Improve error message for missing registrations.

hackerkid/zulip,kou/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,hackerkid/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip