fix HTTP MR error reporting

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.
Merge pull request #639 from branch 'bwf-mr-http-errors' into develop
WIP in safety commit before I get on a plane / through security
WIP mid-atlantic commit of working Proof Of Concept for Maps
WIP PoC for GETting CRDT values as JSON
Use singular for type names in output
Add context to operations

Add pre and post local replica merge operations
Correctly identify the precondition_context fun in export list
Remember to base64 decode the context
Merge branch 'feature/rdb-crdt-general' of git://github.com/basho/riak_kv into feature/rdb-crdt-general