Resolve a series of match warnings.

Resolves:

riak_kv_console.erl:320: Expression produces a value of type ['ok'], but this value is unmatched
riak_kv_console.erl:336: Expression produces a value of type ['ok'], but this value is unmatched
riak_kv_console.erl:349: Expression produces a value of type ['ok'], but this value is unmatched
riak_kv_console.erl:643: Expression produces a value of type 'ok' | ['ok'], but this value is unmatched
Ignore legacy_gossip related code path.

Even though legacy_gossip still exists, the code path is hardcoded to
false, so ignore dialyzer figuring out these sections of code will never
be reached.

Resolves:

riak_kv_console.erl:152: The pattern {'error', 'legacy_mode'} can never match the type {'error','is_up' | 'not_member' | 'only_member'}