Handle search engine returning status objects to the API

This is a followup to I54ab5207, which allowed search backends
to return status objects. We handled it in Special:Search, but
the API was overlooked.

Bug: 47353
Change-Id: Ib7abe8d5bc19d27c472053602c0e43cb1301b078
Merge "Handle search engine returning status objects to the API" into wmf/1.22wmf2