stream_create: Fix crash on stream creation error.
stream_create: Fix crash on stream creation error.

Commit a9ca5f603b7fbb054f57338e260cbf771a94b2ee (#15863) incorrectly
translated this; stream_name_error is not a jQuery object.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip