Today we often encounter users that are confused by the behaviour of
calling `GET _cluster/allocation/explain` without a body: it _seems_ to
work, but it explains a random shard, and if this isn't the shard
they're thinking of then it's unclear how to proceed.
With this commit we add a note to the response when a shard was randomly
chosen indicating that it is possible, and possibly useful, to explain a
different shard. We also adjust the exception message in the case when
all shards are assigned to indicate why it's an invalid request and what
to do to make it valid.
95edc6deb2