This function was building a list, and then
iter-izing in during return.  Since we return
an iterator, we might as well just yield as we go.

This function could get zero partitions from its call
to `rpk topic describe`, and would proceed immediately
without waiting for anything.

We expect to always be called for a topic that exists,
so throw an exception if describing the topic doesn't
get us any partitions.

This fixes commit 8355e4e

quiesce_uploads would fail silently if you called it for
a topic that didn't exist: that's fixed in the previous
commit.

This commit fixes the underlying issue, that we were
passing a string instead of a list of strings, so
quiesce_upload was trying to wait for each character
in the string as if it was a topic name.

Fixes redpanda-data#8496
Fixes redpanda-data#10655
Fixes redpanda-data#9629