Fix timeout handling for key and bucket listing via HTTP

Commit 32060f0230b00d3b6ecd6ac7fe4a6b4f71744d82 changed error handling
in riak_kv_keys_fsm and riak_kv_buckets_fsm to return error tuples
instead of just the error reason term. This broke the error message
expectations for timeout messages in riak_kv_wm_keylist and
riak_kv_wm_buckets. This change updates the error handling in those
modules to be consistent with the current form sent by the fsm
modules.
Merge branch 'bugfix/http-keylisting-timeout-handling' into develop