alternator: fill UnprocessedKeys for failed batch reads
alternator: fill UnprocessedKeys for failed batch reads

DynamoDB protocol specifies that when getting items in a batch
failed only partially, unprocessed keys can be returned so that
the user can perform a retry.
Alternator used to fail the whole request if any of the reads failed,
but right now it instead produces the list of unprocessed keys
and returns them to the user, as long as at least 1 read was
successful.

NOTE: tested manually by compiling Scylla with error injection,
which fails every nth request. It's rather hard to figure out
an automatic test case for this scenario.

Fixes #9984

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla