Code quality: NameOf and ValueOf should return errors

[paralin]

Throughout the code there are calls to ValueOf() and NameOf() on the QuadStore. The QuadStore currently returns a single value for these functions without an error, but internally the QuadStore might run into some errors when processing the value. This hack is clearly identified in the code: rather than return the error, it's just logged, i.e.:

clog.Errorf("Error in NameOf, context is nil, graph not correctly initialised")
return nil

If the QuadStore has an error while processing ValueOf or NameOf, the returned value will be nil which might lead to many types of undefined behavior - such as queries returning "nil" for one field of a Quad.

[paralin]

Similarly QuadDirection should return an error

[paralin]

I refactored the code to add error returns values to those funcs: https://github.com/cayleygraph/cayley/pull/959