fix HTTP MR error reporting (cherry pick PR #639)

These case clauses haven't matched since the 1.3 release, if my
testing is correct. This patch reinstates use of
riak_kv_mapred_json:jsonify_pipe_error to send useful JSON to
an HTTP client.

This is a cherry-pick of commit 41890a93439bce681e25022b43623e781196c542
from pull request #639.