test: raft: randomized_nemesis_test: `call`: handle `raft::dropped_entry`
test: raft: randomized_nemesis_test: `call`: handle `raft::dropped_entry`

This exception happens when the leader stops being a leader in the
middle of a call. Expect it to happen and return it in the result
variant.

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