Implementation does not return a 501 for non-implemented query functions

[andrewbonney]

?query.rql and similar should return a 501 at present. 'Basic' queries for attributes which don't currently happen to be part of the schema should probably result in an empty results list, otherwise the query side needs to be more aware of schemas.

[andrewbonney]

I've put a quick fix in for the immediate 501 issue. Leaving this open however as there are other related things which ought to be dealt with.