import: Fix propagation of subdomain error messages.
import: Fix propagation of subdomain error messages.

The previous logic would provide a very confusing error message if the
subdomain was already in use.

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