Make order_by not drop get_all.

Also adds tests which test this.
Pass rcheckable_t *s rather than backtraces.
Fix a bug with referencing sindex_id rather than _sindex_id.

Also renamed _sindex_id to make this mistake less likely.
Redo an error message.
Improve an error message to reference indexes.
Add tests for unindexed order_by followed by indexed.