FTS fixes of highlight, query string and errors handling and parsing

During the REVIEW phase, the following problems have been identified with FTS RFC that affect the implementation:

• StringQuery should be QueryStringQuery
• there is an additional plaintext error case: HTTP 412 when a consistency is requested but cannot be fulfilled within the timeout
• highlight style is actually optional
• errors is not an array but an object, and when it is present, hits can be null