Refetch an HS's desc if we don't have a usable one

Refetch an HS's desc if we don't have a usable one

Previously, we wouldn't refetch an HS's descriptor unless we didn't
have one at all.  That was equivalent to refetching iff we didn't have
a usable one, but the next commit will make us keep some non-usable HS
descriptors around in our cache.

Code bugfix on the release that introduced the v2 HS directory system,
because rend_client_refetch_v2_renddesc's documentation comment should
have described what it actually did, not what its behaviour happened
to be equivalent to; no behaviour change in this commit.